More slow-mo sequences (like the end of MHC) in SP, and Captain Price. Other than that, there's not really much to improve on.

In MP, I want my M16 w/ red dot sight back exactly how it was. Tone down Martyrdom and the underbarrel GL. Less sniper maps (in the vein of Overgrown & Bloc.) Eleven assloads of new challenges (not weapon-based get X kills, but creative ones.)