If you've played Project X-Zone you know what I'm talking about, but for those who haven't look at its wiki.

The rules are simple, but the roster may not be as big because I don't want to keep track of that, but the rules are simple, one user gets two heroes and one villain (Villian optional), the heroes and villain that the user plays as, need to be from the same company like Chun-Li (Capcom) and Morgan from Darkstalkers (Capcom) Would be fine but, say, Death from Darksiders would not be played by the same person whose playing as Batman. 

And interesting thing about X-Zone is that characters from the same company would know each other  even if they arnt from the same game/show (Yes I'm allowing tv characters), I'll use shows as an example this time, If someone was playing as Peter Patrelli (?) from Heroes (NBC), and someone else was playing as Olivia Benson from Law and Order:SVU (also NBC), Peter might know Olivia from the news, and vice versa, not only because they are both from NBC, but they are also both New York locals.

If people have had previously been on a crossover of any kind they'll know each other from that event. The easiest bridge is Pac-man, surprisingly, he is in Street Fighter x Tekken, and Smash Bros. That would mean that pac-man could vouch for Nintendo characters to the Capcom and Tekken characters and vice versa.

Any questions?