Funnily enough, I had designs on doing something like this. It was going to be a Kingdom Hearts/Yamibou-styled multiverse where the players' character(s) hailed from different universes. These would be original or canon, depending on what the player felt like going with. And basically, we'd get together and go on adventures and stuff.

Anyway, this would actually be a good place for me to play with some fanfiction concepts I've been toying with:

[list]
[*]Huey Freeman from The Boondocks becoming a Samurai/Jedi. He might even get his hands on the darksaber, since it's vaguely katana-shaped, which I think goes with this theme.
[*]Alternate Universe versions of (Future) Trunks and Gohan from DBZ, based on their TFS depictions. I basically wanted to extrapolate on their abilities. Such would include Trunks either channeling ki through weapons, or outright shaping his ki into weapons. Meanwhile, Gohan could learn alternative use for ki, essentially turning him into a sage not unlike those seen in Naruto, and cultivation fiction. I'm tempted to throw Tien in here too. But, I'm not sure what I could expand upon with him.
[*]Superboy, based on his Young Justice depiction (since that's what I'm most familiar with), and he'd learn to use Hamon, and similar techniques. Essentially bringing together his human and Kryptonian traits to make him more [i]complete[/i] in his own way.
[*]Shinji from Evangelion, though he'd be based more on his manga depiction. While flawed, I find manga Shinji much more likeable than his anime counterpart. Not sure what I'd do with him though.
[*]Tails, or really nearly any character from Sonic. Basically, just an excuse to play around with their characterization and abilities and see what I can do with them
[/list]

Of course, I have original ideas. But, many of those are still in the works. Also, I don't intend to play [i]all[/i] of these ideas. Or, at least not all at once.