Someone is already converting Pathfinder for an MMO, and another company is doing World of Darkness. I wish both those efforts good luck and hope they produce some excellent games (and before anyone says No, neither of those is going to be a WoW killer) and there are still rumors floating around about a 40K mmo. Plus I've heard some talk of a Shadowrun game as well
There are quite a few other games out there that would be excellent sources for a new MMO.
For my part I would love to see MMOs based on CuthulhuTech or Eclipse Phase. Both are dark horror SciFi settings. Eclipse Phase even has character respawn built into their setting as players can upload their EGO into new bodies or make back ups before they go on dangerous missions.
So what Table top RPG games do you play that you think would make a good MMO.