A Tales series MMO!
Mainly just because I love the series but the race and class work has pretty much already been done, as has weapons and armor, obviously a new story would have to be created but you could base it in one of the already established worlds, Tales of Symphonia would make a great MMO universe, you already have 2 factions (Sylvarant and tethe'alla).
It could use similiar 'rifting' open world pvp like Aion (moving from 1 world to the other in certain small mana rich areas), pvp story incentives are simple, you are trying to save your respective world.
I was a big fan of aion so alot of my ideas would draw from it but again you already have a 3rd NPC faction, Cruxis and/or the desians.