Actually there is a high variety of good possible systems I could imagine for a MMORL:
- all people meet in a town and can form a 'play group', then that 'play group' plays through a random dungeons (well that actually already exists and is called Phantasy Star Online)
- only trading items and talking is part of the MMO, the actual dungeon is single player
- a "Versus" roguelike where achievements of one player mean more traps/monsters for his opponents
- a really random roguelike MMO where you just keep advancing from place to place, but through randomness you can end up in the same level as another player and are able to cooperate with him to clear the level with less loses (or maybe possibility to fight each other)
- a self-creating world, at the beginning there's only a village, but if the first player ever leaves a map into an yet-unvisited direction, the next map will be created randomly, but will remain like it was created forever (or until a certain event, like the final boss being beaten or whatever). So eventually you'll endless with a huge randomly created world, that has no end no matter in what direction you'll go