capcom did it with marvel vs. capcom. some options include:

quake vs unreal tournament
quake vs half-life
quake vs doom


players can select from a couple or more different characters from each series. the maps are populated with weapons relative to each player's deathmatch universe. i don't really know anything about any other deathmatch games, but i guess shotgun from quake and shotgun from doom would spawn in the same spot, then rail from quake and shock gun thing from ut spawn in same spot.

movement system is determined by character selection. quake has strafe jumping, ut has dodge stuff, etc.

this will give a larger audience of deathmatch games a good reason to pick one game to play.