I have an idea for a co-op game mode for Quake Live. It's called Strogg Invasion, and it's similar to the Invasion mode found in UT2004. The main difference, however, is that players will work together to fight against legions of Strogg troopers. (The Stroggs would be from Quake II, but remade for the Quake III Engine.)
After every four waves, there will be a boss battle against a Strogg superboss that can sustain fire from multiple enemies at once. (Let's use the Makron as an example) The boss will appear a few seconds after the fifth wave starts, attacking and wreaking havoc with the rest of the Strogg forces.
After the boss fight is over, the next set of four waves will consist of stronger enemies. The game will last for a total of fifteen to twenty waves of increasing difficulty. (Twelve to sixteen standard waves, three to four boss battles.) After the final boss is defeated, the match will be over.
Well, that's all I've got so far. Feel free to comment and toss in some ideas of your own.