Congratz. That's one of the best single player FPS campaigns of all time. I remember the first time I ever loaded it up, back in 2000, and I was blown away by the brilliant colors after having played Quake and Quake 2.
If you enjoyed playing the botmatch deathmatches, you might enjoy the online multiplayer Unreal Tournament games. I think your best bet in those regards is UT 2004 where you can still Onslaught servers (has vehicles) that fill up to 32 players each night as well as Invasion Monster Hunt RPG servers which you can play 24/7.
(In Invasion-RPG the server remembers your name and IP, allowing you to level up a character over several months time. There are things like magic weapons, etc. It's non-competitive but you can work with other players to do things like share weapons.)
Note that the original Unreal Tournament (1999) is widely regarded by many people as being the greatest online multiplayer FPS game of all time. In fact, people are still playing it to this day with a group of die-hards organizing 5v5 clan-match-like games over IRC. You can still find many populated public servers for it but they are mostly running various types of mods such as low-gravity Instagib. (Maybe you would like that, who knows.)
The Unreal Anthology for $7 looks like a great deal! Get that--you get Unreal 2, UT99, and UT 2004.
http://www.gogamer.com/_stcVVproductId6442209VVviewprod.htm?ref=dynamitedata.com