You could always try EvE (www.eve-online.com
). Its a spaced based MMO with a very strong PvP aspect (If you have a go then look me up in game - Delphi Denon - Im the CEO of a major corp within a large alliance, we have lots of PvP ops and you can come on them as a noob if you join up!). Failing that maybe SWG would be worth a look due to the more recent updates looking at a more FPS style.
Also for single/multiplayer goodness (well non MMO) look up "Hellgate London" its being made by the guys who left blizzard after Diablo, and its the spiritual sucessor to Diablo 2, but in a post modern world where a Hellgate has been open in London. Personally I think that will be fantastic when it comes out in the summer!