I once saw someone who made a mock Left 4 Dead poster, for a campaign called 'Still Alive?'
Seeing the Survivors fight their way though the Center would be interesting, especially if there were Infected subjects (Rat Man and Chell could be like Jimmy Gibbs Jr. in Left 4 Dead 2?). And then have to fight GLaDOS.
-shrug-
Other than that... I dunno, Metroid and Half Life 2. I just want to see who'd win in a one-on-one fight, Samus or Gordon. (My money's on the 6'3 mutant in a powersuit.)