Better drift physics, preferbly from the Underground series. I have no idea why they changed them for Carbon, the feel was horrific! And if they bring back the whole crew racing thing, I can only pray they'll make them smarter so they won't fuckin slam on the brakes right in front of you when you into a turn's apex. So damned frustrating. Better music too, but I'm guessing if Carbon 2 comes out on the next gen consoles, that won't be a problem.
And a MUCH longer storyline. Carbon was disappointingly short. And online play for PS2 for christ's sake.
I wish I had a Xbox 360 so I could get Forza 2. It combines Gran Turismo's simulation with NFS's customization. Thats all I could ever ask for in a racing game.