I have always wanted some manner of Multiversal RP without limitations, but even I understand that it's nigh-impossible to regulate.

(From an Arena standpoint at least.)

You'll have a character like Superman floating about dimensions when suddenly he stumbles upon Captain Jack Sparrow over here, then they get into a heated drunken discussion. Two and a half hours later they're either still duking it out because Jack Sparrow is now a super saiyan, or Superman has leveled half of Manhattan for some ungodly reason.