The Watchers think that one of the Knights of the Cross is an Immortal because, hey, running around with a sword and all.

Saucy'verse Xander, after he sacrifices himself to close the portal, finds himself younger and with binocular vision in the Dresden'verse, but he doesn't have long to think about it because only a few seconds after he appears he hears gunfire and sees some guys in suits fending off...something.  Which he recognizes as never nice, so there's no dilemma in his mind about which side to support (hey, it's a legitimate problem--sometimes there are asshole humans doing bad stuff to good/neutral demons, and those situations don't always that way at first glance), and he swings into action, literally, considering the sword cane.
One of the men is John Marcone, who realizes pretty quickly that Xander needs wizardly assistance, where I don't know what happens other than he can't be sent back/send himself back, and has to stay in the Dresden'verse.  Something about the Laws, since Xander has to learn them and they have a different magical government (so to speak) back home.
Not sure how Xander gets along with anybody else, but he and Marcone get along like a house on fire.  Maybe he's not ecstatic about the whole mob thing, but Xander understands Marcone's position as a leader setting the rules and trying to keep his people in check, and occasionally having to do distasteful things because of it, so they hang out when Marcone's not busy, and Xander doesn't comment (morally, anyway) on Marcone's business.  If he ever meets Bob, he's reminded strongly of Anya.
Eventually he starts to come to the attention of people outside of the usual allies, because he's got skills and uses them when the situation warrants it, and he does hang around with some walking targets.

AU.  Pre-canon (I'm thinking back when Charity had her magic stage, but I don't know Harry and Charity's relative ages, so it might not work that way), Charity and Harry met and became friends and stayed that way until they lost contact.  Fast-forward to Harry being introduced as Michael's friend the wizard.  Charity has mixed feelings towards him, but she's nicer than in canon--he might be a wizard, but he's Harry.

Atlantis Five-0

Basically, Hawaii Five-0 set in Atlantis, in the Atlantis University verse, maybe.  Steve's transporting a prisoner for one of the allies (his job means he mostly works outside of Atlantis), when he gets radioed by the guy who's got his dad, and attacked, which he survives, and then he's all, "he's on Atlantis! D:" and goes back home.  It's at least a generation since Atlantis was resettled, and there are a lot more people than there were originally, so it's possible for the guy to lose himself in it.
The Stargate Program's gone public on Earth, and some Tau'ri are allowed onto Atlantis for various reasons.  Step-Stan gets a job on Atlantis, taking Rachel and Grace with him, and Danny follows even though he has to jump through all sorts of hoops to do so (the fact that he's doing it for Grace helps a lot), and very much does not assimilate.  Steve's parents moved to Atlantis when he was young, but he's not native Lantean.  Chin and Kono are.
Obviously, some things have to be changed because the culture's so different, but it could totally work.

Shawn Spencer actually does have psychic powers, and always has, but back when he was a kid Henry decided to push him into becoming a cop, with training as in canon.  Could go either way, either Shawn's aware of it all along, or Henry keeps it a secret from him and convinces him that it's just photographic memory and extensive training.  Because I've seen a couple of "Shawn gains psychic powers" fics, but I don't think I've ever read one where he's always had powers.

Fantasy; original, crossover, or timetravel.  For whatever reason, a kid who's had a very good and thorough education in the use of his/her powers (they've had them since birth, and since their parents were involved in the supernatural they had the benefit of learning how to use them along with everything else like walking and talking; by this point, they use their powers very fluidly and naturally) and an adult/older teen who's new to their powers, clumsy with them and possibly afraid, have to live/travel together.  Post-apocalypse, maybe?  Not many other ways to fling together an adult and child who don't know each other previously.  The adult has to do all the adulty things--protecting the kid, making sure they have enough food, etc.; but the kid has to teach him what he needs to know, because the situation's dangerous enough that they need to use their powers to stay safe.

 The usual Star Trek XI crew, minus Spock, were all on Tarsus IV when Kodos made with the killing, most of them kids at the time, and managed to survive under the radar, with the bridge crew emerging as the leaders because of their age and the fact that they were the ones with the ideas.  Bones is there with his wife and Joanna, but ends up in Kodos's dungeons or otherwise helpless while Jocelyn sacrificed herself to save Jo, who ended up with the kids; eventually he meets up with them, all skin and bones.  Scotty and Keenser are there, too, fresh out of SFA, and when they first meet up with the kids barely trusted at all because the kids have recently learned to distrust authority, and as Starfleet they're authority of a sort.

Most of them go by one name or nickname, especially as time goes on and the younger ones forget their last names and they get more angry at the families that didn't protect them.

A few months in, the other colonists get rescued by Starfleet, who don't realize the kids are there because they're hiding (again, the distrust thing), and it's years before anybody comes back after that--Starfleet again, with Pike.  The kids have survived and done relatively well for themselves, even studying as much or more than they would have if the school for the gifted had still been there, if a bit more unevenly; they have all of the materials, and a lot of them were always motivated to learn, much less now, when a lot of it helps them survive better.

Pike comes in, finds them, and is horrified that they were just left there and wants to get them back to their families...to which they say "hell no".  Eventually he talks them around to a compromise, terms involving at least them leaving Tarsus, and convinces them that the only way anything's going to change is if they change it themselves by finishing their educations and joining Starfleet.  Of course, he thinks they've been on Tarsus for years without education, so it'll be a while before he has to deal with them doing that, since they couldn't possibly pass the admissions tests.

Not so much.  For the next few years, they flood SFA with their admissions, making people look at them funny for only using one name, and for the most part remain together as one big clique on campus.  Well, at least they'll be broken up by their duty positions, Pike thinks.  ...We all know how that went.  They're not in charge of the Enterprise to start with, but JT hooks back up with Scotty and Keenser and takes it over; immediately, the Tarsus cadets snap to higher efficiency and awesomeness than ever before.  Spock is fascinated.

Possibly Tarsus is classified; either way, nobody knows that the kids are survivors unless they were there or have very high clearance.

ETA 04/18/2011--Girl!JT, who is at least as twice as badass as any guy, and BFFs with Uhura except when she isn't. And since Cupcake was totally on Tarsus (how do you think he got the nickname?), the bar scene is totally different. Though, if the plan is to get into Starfleet, I don't know why JT wouldn't be with the cadets. Not too hard to come up with a reason, though; maybe her record's bad enough that the recruiter is all "uh...how about no?", and of course Pike doesn't even look at her record. Hm, maybe not Pike finding them, so there can be the nasty surprise. Or he just didn't remember any of the kids' names, since he didn't know (of) any of them except JT, and didn't meet her, and they all look so different...lol, or he thought JT was a guy at Tarsus, and JT didn't tell him to call her JT in the bar so he didn't have any clue. And JT's crew love her, but Starfleet...not so much. She's got the cult of personality going, and only follows orders when she feels like it, and is a big damn hero, and swears a lot and in general doesn't project the image Starfleet wants to project. But they can't do a damn thing about it.

Bones randomly ambushes the Tarsus people to give them checkups and hyposprays, and they just go along with it because they trust him.

JT learned early on that it was less unsafe to be seen as a boy than a girl; as did the rest of the girls, but JT's better at passing than, say, Uhura. Besides, they all kept their hair short to prevent lice, anyway. JT just never grew hers back or started dressing like a girl after the rescue--she was a tomboy beforehand, anyway. She's not pretending to be male anymore, but with short hair, bone structure that passes for male although she wouldn't look bad as a girl either, a pretty flat chest, and a personality that most people take as masculine, most people don't go around asking. Mostly by accident, she ends up only having sex with people who both are bisexual and for whatever reason don't tell the rest of the Academy that she's a girl. Her first few roommates refused to remain her roommate; eventually the Academy agreed to let her room with Bones because he was the only one who would take her.

The more I think about it, the more this seems like it should end up JT/Uhura. Kicking ass together in the beginning, BFFs for the longest time afterward when they're not fighting, and finally they get together.

Methos (or other character who's not quite human/has special abilities) is captured by the NID, who do Bad Things to him because they know about his abilities.  The SGC mounts a rescue, saves him, and asks him to join them because his abilities would be useful.  But he's all "not just no, but hell no" because they might have rescued him, but they're part of the same thing.  Of course, they let him go because they're the good guys, and he disappears completely.  SG-1 keeps getting called in to investigate/to be accused of the destruction of NID bases.  There's a growing suspicion that Methos (or, well, whatever identity of his that they know of) is responsible; they find him destroying the last base.

Confronting Duncan

Duncan stopped mid-sentence, tensed, and turned to the door of the bar, alerting Joe to the presence of another Immortal. Who would it be this time? Amanda? Methos? Another of Duncan's Immortal acquaintances? Somebody who would inevitably try to kill him? Maybe the years of being so close to Immortals had jaded Joe, but it seemed like it was never good news when an Immortal Duncan didn't already know came to town, and usually when one he did know came to town, too. At least they didn't tend to see Joe as a worthwhile target. He was just Duncan's bartender, after all, not somebody who could make Duncan fight a Challenge by being threatened, not like it had been with Tessa Noel.

The Immortal who walked in through the door wasn't anybody Joe had expected. For all their faults, the Watchers actually were fairly good at keeping track of the Immortals, and as the Regional Supervisor, Joe was supposed to be notified of any Immortals coming into the area. The number of Immortals moving in and out of the region lately was not inconsiderable; Immortals always had been more mobile than mortals, and modern transportation had only increased that movement. But still, Joe was certain that this Immortal had not been one of the Immortals he had been notified about. Even if the eye patch was false--and he didn't think it was likely that an Immortal would give himself an identifying mark like that, but then again there were Immortals who made themselves more identifiable, and at least an eye patch was easily discarded--none of the Immortals who should be in the region matched his stats. Either some Watcher had forgotten to send in his report, or this Immortal didn't have a Watcher.

"Duncan MacLeod of the Clan MacLeod," Duncan introduced himself. "Why don't you sit down and have a drink?" Ah, the subtle method of ascertaining if the Immortal's intentions were peaceful.

"Duncan MacLeod?" The other Immortal asked, his voice cheerful. "Just who I'm looking for!"

"Oh?" Duncan asked, relaxing at the Immortal's tone. "Why's that?"

"You killed three of my students," the other Immortal said flatly. "Now, I know the Watchers will cover the bar if I take your Quickening in here, but it's only common courtesy to go outside for a Challenge, so if you don't mind..."

"What...?" Duncan couldn't make a full sentence, he was so stunned. Joe took over.

"You know about the Watchers?" he asked, seeing another reason the Immortal might not have one.

"It's kind of hard to miss when you're being stalked," the Immortal said. "Or, well, when other Immortals are being stalked, when you're hanging out with them, since I only picked up a Watcher a couple of times. But it's cool. A bit creepy, but since most Immortals try not to leave much of a legacy I guess it's the only way they're going to be remembered. And it's not like I can't lose a tail when I get one, just like every other Immortal with sense."

Joe had to snort at that. Even the most conservative estimates said that most of the Immortals still alive had Watchers, and hadn't even noticed them, much less lost them.

"Well, it's not like most Immortals have any sense," the Immortal said with an eye-roll. "It's all The Game this, and 'oh, my insulted honor' that, and that's not even counting all the stupid debate about what being Immortal means, and how we should act towards mortals. It's all so stupid." He looked back over at Duncan. "Is he going to stay like this all night? I mean, not that I have anything else to do, but shouldn't we get this done before your customers start to arrive?"

"I think he's still trying to figure out who he killed that would make you want to kill him."

"He kills that many people?"

"Mostly he only kills bad guys."

"Bad guys? What the hell does that mean? Just cause a guy's done some bad things in his life, doesn't mean he should be killed!"

"Mostly, they try to kill him or somebody else first."

"Whatever happened to trapping people in a well until they've learned their lesson, huh? Why isn't that good enough for the kids these days?"

Joe gave him a strange look. "That's how it's been as long as the Watchers have been keeping records. And what about you? Didn't you just Challenge him?"

"That's different. You don't mess around when it's family, you send a very clear lesson to anybody else who might be thinking along the same lines. But you didn't say anything about *his* student getting killed, so he's got no excuse for killing *mine*."

"Look, I don't know who your students were, but they probably attacked him first."

He opened his mouth to reply, but at that instant Duncan stiffened again at another Immortal's presence. The other Immortal was smooth about it, though; if Joe hadn't had Duncan to watch, he never would have realized that another Immortal was approaching until the door swung open to reveal Methos.

Methos froze as soon as he saw the other immortal, and Joe groaned mentally. If those two had a history...he didn't want to have to write the termination report for Methos. But although Methos looked dismayed, he didn't look alarmed. "Beer," he said to Joe.

"He's mine, Adam," the other Immortal said. "They might have been your brothers, but they were my students."

"I killed Silas," Methos said quietly. Joe had to hold himself back from interfering. If the Immortal was so willing to kill Duncan, he wouldn't be any less willing to kill Methos. But it wasn't his place to interfere. And besides, Methos was still...not relaxed, exactly, his shoulders were tight with tension, but unafraid. It made no sense; Joe had to be missing something, and not just the fact that this Immortal had apparently trained the other three Horsemen.

"Oh, Adam," the Immortal said, sympathy in his voice, completely different from his reaction to Duncan. "Why?"

"Kronos and Caspian...they were rabid dogs, unwilling and unable to change their ways even after so long, and you know Silas always went along with them..."

"They hadn't changed?"

"Upgraded their methods, but they had the same goals in mind, and I don't think even you could have changed their minds."

The Immortal smiled sadly. "You did good." He turned to Duncan. "I release you from my Challenge. If Adam says it was necessary, it was necessary."

"Stick around?" Methos asked. "I'm sure Joe's full of questions, and you can always ignore MacLeod when he gets preachy."

"Sure, I've got some time on my hands now that I don't have to spend it recovering from a Quickening."

After Replacing SG-1, mini!Jack returns to the SGC.  Everything has changed.  Basically, things are shitty.  People aren't there because they want to be there, they're only there because they believe in the mission.  And the Legends going on halfway creep him out because even though they're true, the *way* they're said...People are smart enough to realize who he is, although they don't say anything out loud, and he keeps getting these tiny presents--stuff slipped into his locker, the last piece of pie, whatever.

As a prank, an Immortal's friends (who don't know about Immortality) stick him in a morgue while he's passed out, then panic when he disappears rather than coming after them swearing revenge.