Hey, i wrote a massive post before but my comp gave out before i could confirm it:(. Ill try and wrote what i did before.
What i would like to see, is a system where special events occur, heres an example.
One of group of evil players, a devil worshipping guild, wants to summon (insert vowel filled unpronouncible name), the evil archdevil. Everyone knows because it will be a world changing event, maybe the sky could turn red or something. Some ritual may be required or some other system (perhaps a tribute system? or both) where the summoning players have to aquire certain items, find a forbidden tome or something.
Anyway, lets say our evil players do manage it, then complete the ritual. A Devil is spawned with neigh on apocolypic powers (i.e. it is almost beyong killable), Thus requiring a holy crusade resulting in an epic battle.
Anyway, this is an extreme example, a more mundane thing may be for a player to become a vampire, a mayor of a town etc.
Anyway, the \"Why\" to this wish is quite simple. Every other MMORPG promises many things but is always a let down, for example in UO, the most one can hope to accomplish is a moderate sized house (very moderate, due to a housing bug, when housing became available, everyone had a deed, everyone got a small house). Sure you could go and see a dragon, and get mauled, some people could tame them only to have them disappear, Noone could ever achieve great power.
Also, If you make these extreme quest objects Rare (and by Rare i mean Rare no constantly finding them in one place) and forcing people to have to work together for them (all classes, no extreme nerfing of one class while one class was almighty, like the wizard was uber class in UO where as everything else was pretty much useless).
I realise this may cause unbalance (if someone summoned a devil, everyone who tried to fight may be killed) but this could be rebalanced by giving the retaliating players some help, for example if the devil team were winning for an extended period of time, give the good team (note team, i dont think free agents should get any help, although this may conflict with players who wish to play the part of assasins).