Co-Op SWAT?
One of the great things about classic DOOM was the ability to play with your friends at a LAN party instead of against them. It also let you have a smaller group of people playing together and still have decent gameplay. (2 v 2 tournaments get lame pretty fast.)
Word has it that the upcoming S.W.A.T. 4 will enable co-operative play for certain scenarios. If the post-game reviews look good, this might be a fun one.