Though I would love a Seiken Denetsu and FFTactics MMO's as well as Pokemon (come on Nintedo, get off your lazy ass) There are a few others I could think of.
A friend and I are actually working out the details of an Occult MMO,The main focus is empowering the player to actually affect the world around them.
Besides that... I wanted to make a MMO about pulling off intricate heists. You create a thief with special talents and basically work through an underground network to find partners to help you pull off amazing heists. Think of it as a cross between Oceans 11 and Lupin the III. You would need to scope the place out, check the security systems and find people with hacking skills, make-up and disguses, weapons, stealth, lock picking, and even explosives. The missions you would be challenging and sometimes amusing.