In his free time, Anderson is a superhero (I'm thinking a Superman analogue, but whatever floats your boat), which naturally makes him sleep-deprived and distracted, and maybe it's hard for him to tell what humans can and can't see/notice. Sherlock knows he's a superhero, but doesn't see that as an excuse or even a reason for his behavior. He doesn't say anything about it because it's so obvious that it's boring--Anderson doesn't even wear a mask, for crying out loud! ...Nobody else realizes that Anderson's the superhero.

Worst-case scenario for a sex pollen type scenario: it's not a pollen, but a virus which mutates very rapidly, so it can be caught again and no cure can be developed. And it gets transmitted as easily as the common cold, maybe has an incubation or contagious but no symptoms period. Obviously would require lots of research about disease transmission.

Back in the day, Methos was Egeria's host and the catalyst for her change of heart. Post-Horsemen, even though that means changing the date that they rode, so he knows the temptation and appeal of power and has gotten over it. So they Tok'Ra around for a while, then get caught, Methos "killed", Egeria imprisoned. Fast forward to the present; for some reason, Methos is with SG-1 when they find Egeria.

Dexter Peggy Sue: Dexter wakes up back as a child/teenager (before Harry died), with all of his experience with life and killing. Not to mention years of seeing and getting advice from hallucination!Harry, which doubtless does not match up entirely with what real!Harry would say. And experience pretending to be a normal person. And, you know, the fact that killing animals won't cut it anymore.

In the myth of Er, Odysseus decides to become reincarnated as an ordinary person. So...what if that ordinary person is John Watson?

So, one day God says to Joan, "I need you to change your name/get a really good fake ID and go to Miami." The name on her ID is Lumen Pierce.

Methos was in Grantville when the ring of fire happened, either with or without the others (this could be done with almost any immortal, but if I did it I'd do it with Methos; Duncan would be less interesting because in 1632 he was still a baby Immortal...okay, that might be interesting, too). And he knows that younger!Methos is going to come to Grantville, as well as probably a shitload of other immortals, because it's interesting and completely new. But what's he going to do, leave the only real civilization that exists?

Sometime before he meets John, one of sherlock's experiments goes horribly wrong and now he's a zombie. Those body parts? Not (just) for experiments. And he doesn't eat a lot of normal food because it does nothing for him. Nobody knows except Mycroft. But then, somebody finds evidence that he eats the body parts.

Theseus runs around, defeating bad guys by turning their own tricks back on them. So does Jarod. So...what if Theseus was a Pretender?

So, I just read a very short X-Men/Highlander crossover where Joe was a mutant. Which would be really interesting to explore more in-depth.

Admittance to Hogwarts is technically based not on age, but on some whacky magic milestone. It's just that, because its students have all been human, that milestone happens at 11 for all of them. For Immortals it happens after 5000+ years. Methos gets an invitation to Hogwarts, and decides to go because it's something new.