As much as some may grumble, my immediate thought would be to use 4th edition, since the select-a-power chargen system is quite adaptable to most situations, and the combat system, for all its faults, is fast and breezy. Would pathfinder or gurps be a better system in terms of raw mechanics? Most probably, But not everyone knows them and I'd want to avoid having everyone spend an hour learning a new system from scratch to play a one-off. 4th is ridiculously intuitive.
The general premise I've been toying with is that our club have all been whisked away to what could be loosely described as the "Meta Dungeon"', where all systems and the worlds they evoke exist side by side and perhaps, sometimes, even overlap. Each table of players must travel to each of the rooms that represent a system and complete one or perhaps two objectives:
1) locate the key that will allow them to escape to the next room, which will of course be fiendishly hidden.
2) defeat the RAID BOSS. or something equally horrifying and also awesome. Corpse looting will ofc be mandatory.
Now, when I say room, I'm being pretty loose here; the cyberpunk game might choose its room to be the top floor of a neon skyscraper that looks out over a city the size of a small country, the Iron realms lads might set their dungeon around a sprawling factory floor of the local khador jack-plant, whilst us Exalted muppets will of course go completely over the top and set our room within the bowels of an active volcano that Is a actually the lair of the Magma God.
See. Just needs a bit of imagination and a shit-tonne of floor tiles
Mechanically the idea is that you have a thirty, perhaps forty minute time limit to explore and solve a complex but self contained dungeon room that doesn't necessarily have to be a room in of itself. Players who survive get points and the chance to roll on that rooms loot table, where they might pick up gear that is appropriate for that world. Players who die simply re spawn in the next game, with no points and no loot. Points are also awarded for raid boss defeat, but it'd be nice if GM's didn't feel compelled to force the fight; the world boss might just be looking on, watching our players, and will only fight if challenged and so on. GM's should feel free to tailor the room to the feel and the flavour of the world they're representing.
At the end of the night, we tally the points of each player and they contribute towards their Table Score. Successful players make for winning tables.
Obviously this needs a lot more work and some serious communication with the our GM's, but if people like the idea behind it ill try and nail something more solid down to work with.
Thanks for reading my mega wall of text.
Dan Stubbs; Exalted enthusiast and talented drawer. Just ask my table.