The Marauders' Map is a horcrux of its creators.

The Leverage crew all got killed in the pilot, but they all stuck around as ghosts and don't realize that they're dead. And they're so awesome at acting that they fool everybody, even though normally it takes decades for ghosts to figure out how to affect the physical world or be visible.

And then the Winchesters figure it out.

When Kal-El landed on Earth as a baby, the Kents somehow got Kryptonian powers and the instinctual knowledge of how to use them. Although it's interesting to contemplate how things might be different in the big picture when his powers aren't a unique thing, but rather a family thing, I'm more interested in a random domestic scene where they all...heat up their tea with laser vision, or something, extremely casual about it

Variation on an earlier idea:

After the most recent fun with thieves disguised as terrorists (which could be any of the movies! :D), John McClane gets a new partner. He keeps getting stuck with the undesirables because nobody else can handle his crazy, so it's not really a surprise that he gets stuck with the new guy...who just moved to New York from the UK. How much of the city will be destroyed in the name of saving lives before John McClane and Harry Potter get split up?

Starfleet Academy (well, technically, everybody, but only SFA or possibly just the students remembers it) gets caught in a time loop before all the craziness. Things, um, get a little bit crazy. Especially since it's one of their emergency simulation days and Kirk's in charge.

I've never seen Joan of Arcadia, but I'm pretty sure that she was a nice and normal girl before god started talking to her. What if she wasn't? What if she was more like Dexter, only faking being normal, and holding off on her darker impulses as long as she could because if she could love anybody, she'd love her family?

Somehow, Luke and Leia ended up getting raised by Han (er, might need some age juggling, there) and Chewbacca. And then, they give Ben Kenobi a ride...

Harry Potter isn't the only new student at Hogwarts who was left with relatives as a baby--Luke Skywalker was, too. In the Star Wars universe, not ours; Hogwarts serves only Tatooine.

One of the Highlander characters on Pandora or as a red shirt, getting killed over and over and over again but with no place else to go even if they desert or whatever. Comedy.