Joystiq posted an excellent roundup of 31 upcoming MMO games. Themes include everything from the standard fantasy and sci-fi fare to wild west, robot, zombie, sex, pirate, and post-apocalyptic themed worlds. Similarly, many of the titles are pretty traditional MMORPG games, but some provide an emphasis on strategy, FPS gameplay, chat and casual gameplay, or other variations.
The games that sound most interesting to me are: