Vacation's Over

A long time before the actual story, a group of heroes (I'm thinking the Scoobies, but it works with any heroes), after years and years of saving the world and becoming immortal, are rewarded by the Powers: a guarantee that the world and themselves will be kept safe and they will be basically blessed, and that they can just go off and enjoy life. It doesn't come too soon; the heroes are all weary and sick of fighting, and ready for an extended vacation.

Fast forward to "the present". A bean counter in the Powers notices they're expending a lot of power on people who, as far as he can tell, do nothing, so he or they cut that power off.

The heroes notice. By this point, they're rested and willing to rejoin the fight, and getting kind of bored now that the power which has been keeping them happy has been cut off. So they go back to the hero business. Experienced and rested, they're better at it than they were the first time around, and bounce around solving problems and causing chaos.

Normalcy

The Dursleys try so hard to be normal and fit in with their neighbors...so how would they act if they lived in a not-so-normal town?

Bellatrix LeStrange is insane and completely devoted to her Lord, so when she has a change of heart about the Death Eaters' values, the only action that makes sense to her is to deage Voldemort and raise him according to her new values.

Sci-fi civilization builds a ship with a field that makes it out of phase with reality so that they don't have to worry about asteroid impacts destroying it. But something goes horribly wrong with it, and they find themselves in another reality, where things are different in significant ways.

The time the Key can be used is actually calculated based on the year of a planet other than Earth, possibly one with a very short year. Optionally Glory isn't able to open a portal. Either way, sometime after Sunnydale turns into a crater, Dawn cuts herself (or is cut by somebody else) and finds herself in another world.

Batman Meets the Key

The insane see Dawn Summers differently because she's the key. Bruce Wayne is insane (you know it's true!). Dawn goes to Gotham for some reason.

He doesn't say anything to her, except possibly if he's in the uniform. He might say something to his colleagues, especially if they're on patrol together(-ish) with open coms, or if he had reason to be worried that she might be a threat to Gotham.

the HP-AU Jeannieverse - the Weasley twins, as an ongoing Academy project, research and write a book about pranks (Pranks: History, Philosophy, and Strategy). They spend a lot of time on research, even at home, which makes Ron not jealous of them, because the Academy looks more like something Percy would be interested in, from his POV (they don't even have quidditch! all Fred and George do is study!). Part of the research involves contacting Moony, Wormtail, Padfoot, and Prongs, of which only Remus replies. Other parts are tracing the history of pranking and specific pranks, the purpose of pranking (bringing happiness), and how far is too far/the ethics of pranking. The book is up to academic standards and gets published.

Xander, Willow, and Jesse meet, make friends with, and are kept away from the darker sides of Sunnydale by friendly demons starting when they're fairly young, because none of them want to spend time at their homes and their parents don't want them at all, much less want them there. They have no idea there are evil demons, or at least demons who are more evil than humans- they see demons as just people, albeit strange-looking people. And then Buffy comes to town and Jesse gets vamped and staked.

Slayer Essence

The First Slayer hadn't been very subtle, nor had most of the Slayers who followed her, in anything other than fighting style. To them, the world was divided between humans to be saved and demons to be slain, black and white with no grey areas, and nearly all of them – especially those who survived a significant period of time once they passed from Potential to Slayer – had been raised by the Shadow Men or the Watchers or whatever they were calling themselves at the time, their lives containing nothing but duty and learning everything they needed to know before they were Called.

Buffy Summers and Faith Lehane were birds of an altogether different feather. They'd both survived longer than most Slayers survived, neither of them with the sometimes-dubious benefit of being raised by the Watchers. Both knew how to take pleasure in life, even while doing their duty. They knew how to be subtle, how to blend in, and that not all demons were evil and not all humans good. They had a wealth of experience most Slayers never even dreamed of getting, from the best of life to the worst of it. Dead, their opinions carried considerable weight.

The Slayer essence changed then, like it had never changed before. The system had worked as it was – the world was still there, wasn't it? – but Buffy and Faith made the rest of the dead Slayers who were now part of the Slayer essence realize that just because it worked, that didn't mean it couldn't be improved.

From then on, Slayers got more dreams from the Slayer essence. Not prophetic – the essence couldn't control when the Powers handed those down – but from the essence itself. Extra training in the girl's weak points, from concentration to fighting style to tactics to demon recognition to having fun. Encouragement. Somebody to talk to. A shoulder to cry on. And while they were awake, a small voice in the back of their heads, giving advice. Of course, the Watchers congratulated themselves when the average lifespan of the Slayers went up.

It was more difficult to communicate with Potentials – since they weren't Called yet, the connection to the essence wasn't as solid as the Slayers' connection, and there were so many more of them that it wasn't worth the extra effort to give them the deluxe treatment the Slayers got. They got dreams, but for most of the Potentials they were vaguer and not as personalized (if they were in a bad situation or had a lot of stress, the essence made an extra effort for them). But mostly the essence influenced them by giving them nudges: to be at least initially interested in martial arts of some sort, to stay in shape, to judge people on their personality rather than their outward appearance, to have fun but not forget their responsibilities.

The Atlantis Expedition assumes that, barring a major miracle, they'll never reestablish contact with Earth, so they plan and act accordingly, by making sure they have what they need to become self-sufficient and raise and educate the next generation to some approximation of their standards, and making friends and allies and integrating some of them into their society. They plan it from the planning stages back on Earth, not from when they meet the Athosians or something like that, because that's a major attitude shift that they'd have to have from the beginning. So, prepared to educate and integrate, they take in the Athosians and make them an integral part of Atlantis, and random other people. Maybe they don't reestablish contact with Earth until (a lot) later. With each of the major cultures they accept, the culture of Atlantis changes, because it's not a "you're just staying in our city" thing, it's a "this is your city too" thing, and while there's a certain amount of picking up Lantean customs going on, the new citizens don't come into it with blank slates: the kids from M7G-677 aren't going to take "you're not old enough" as a reason why not, etc. Some people (Sumner, Bates) would have to be reconsidered, because you assign certain people if your goal is to be an outpost, and completely different people if your goal is to make an independent colony/culture. Sumner and Bates really didn't have the right attitude for the latter, although it's possible they might have had a different attitude if things were different.

And, because they have the means to teach and can always use trade goods and more hands to do the grunt work, Atlantis takes on students and teaches them (for a price, and the students have to work, and it's mostly science and practicals, because practical stuff is what they need, and science is ultimately what they need to defeat the Wraith...and those are the subjects Atlantis has the most people to teach), and becomes known as basically Pegasus's first major private school/university. Possibly the Genii are the first ones offered this, or send a lot of people to Atlantis. The Lanteans are a lot more cautious about the students, because they're not citizens, and although they might try very hard to only let the decent ones in, there's really no way of telling what agendas the students might have in mind. Consequences are dire for those who carry out plots against Atlantis, namely execution and blacklisting their culture from the University (and trade?).

Of course, the inevitable eventual contact with Earth would be...interesting, to say the least. Because, well, Earth sees them as a colony that hasn't been in contact. The Lanteans see Atlantis as home, and it's kind of weird to them that Earth keeps trying to exclude the non-Earthborn Lanteans.

The roles of the characters of X fandom are swapped as much as possible. Obviously some roles are situational or personality-based rather than random enough to swap (SG-1's roles are (military) hard scientist, soft scientist, military, and alien warrior. Some could be swapped easily; others would take major rewriting of backstory and even names- Sam Carter is the former first prime of Apophis...but then what about Jacob? Why do they have American names? Etc.), but for instance I could see it going well with Buffy: Willow the Vampire Slayer! Xander the witch! Buffy the token normal person! (Jenny the Watcher! Giles the Gypsy!)

Watcher Xander 1

They won against the First, but in the end the only one who survived the battle was Xander.

Like the cockroaches they were, most of the Watchers had scurried and hid at the possibility of danger to themselves. And when it was all over, they resurfaced, grateful that they were still alive and that the troublesome Watchers had gotten killed by not throwing their Slayers to the wolves. They promptly sent a team to forcibly warn Xander away from the supernatural.

The team turned up two weeks later, naked and tied up in the middle of a demon bar, physically unharmed (though the same couldn't be said for their mental states) despite the lack of an anti-violence spell. As far as the Watchers' Council was concerned, Xander Harris was never seen again.