Honestly, it is VERY hard to limit this kind of thing, as a lot of the time, people will want to play people with godlike powers, which means you have to start banning characters. Also, certain Universes don't mix well with others. Take DBZ for example. Goku and Gohan will pretty much poop on every other fictional character if forced into a fight.
Another point, is that every fictional character already has a set personality, and some RPers don't realize this, and can't play them correctly, which then other fanboys will pick up on, and bitch about,
Well, the past two Multiverse threads actually handled this very well; the people who played the characters generally were neither OP beyond what was reasonable or acting unlike their character would. The only issues both those threads had was, for the first, the admin disappeared without warning; the second was due to life getting in the way of the admins.
Couldn't help but overhear you guys wanted to find a multiverse RP. My Green Lantern RP might be just what you're looking for. I've designed it so that it's made up of multiple fictional universes. DC, Marvel, Dragon Ball Z, Doctor Who, Star Wars, Star Trek, Alien, Predator, Halo, Gurren Lagann... Hell, I somehow managed to cram Transformers and Sonic the Hedgehog in there. Though all of the stories center around the various Lantern Corps and their members. Feel free to check it out. We'd love to have more active members.
No thank you. Counting mine, I'm already a member of two different RP Groups, and mine could always use a little work. Hell, I still haven't gotten around to editing the rules to say that a Saiyan/Kryptonian Hybrid is off-limits in my group...