Ya know, I have a history in MODing Starcraft, and I could probably MAKE something like that. Only problem is, even with a staff of 10, it would take over a year to make it. AvP2 MOD for Starcraft is STILL in development since 2002. Dont luck out there...
But I believe Half Life 2 would easily be the best to MOD with. I can make levels, while the guys with 3D Max can go at it with the Shivan and Terran design. I imagine it wouldnt be hard to replace the Marines from Half Life with different uniforms. Believe me, Half Life 2 is the easiest to mess with (note the awesome mod: natural selection) and also one of the best. We also have to think about one BIG thing:
The Physics
Holy crap, I have watch the Half Life 2 video over and over and over again, and I must say that simply, the physics are what are the mainstay of this monster, and I think that ANY attempt to utilize it would bring fourth much rejoicing. Besides, I know that possibly SOME of us are getting tired of Space Combat, and want to bring it to the GROUND baby. Yeah!!
One mission I had in mind was the first one, when the GTVA is pulling civilians out of Cygnus Prime. The planet can serve as an awesome battlefield...