I second that Warhammer 40k Now dammit.
Unreal Tournament MMO w00t
A Final Fantasy MMO with proper side vs side combat system.
Civilzation MMO most definitly.
A Sims MMO.... no dont look at me like that. I would honestly like to see a Sims MMO implemented like Animal Crossing. Dont go over the top, just tie in the Sims 2
A WWII MMO. Done properly. Ok WWII isnt an IP but meh, you could be forgiven for thinking it. None of this warcraft nonsense. You can have it take place just after the DD landings. Both sides in a stalemate. You can be sub captain, a fighter pilot, a bomber, a tank, a grunt soldier etc.
You could start off having to be a soldier or bomber crew or navy recruit, work your way up over a few weeks to commanding a squad, a ship, a fighter wing etc. Player acheivements effect the battlefield until one side losses and it resets. Both a Pacific and Atlantic server... ingenious.
/me hurries to finish his Programming degree before somone steals his uber if unimplementable idea.
All in all any IP into an MMO would be sweet if it isnt another chance best point and click em up fest that is about as much a computer game as snakes and ladders online.