What I miss most in DR2 is the hill climb mode of DR1. I really hope it will return in DR3 with some new locations (unfortunately, Codemasters don't have the license for Pikes Peak anymore).
Otherwise, I'd like all the DR2 locations to return without having to buy them all separately again ...
Also, I'd like locations with sand tracks (Africa maybe?).
I'd also like a livery editor as in DiRT 4 (preferably a bit more sophisticated). Also, the game should be more mod-friendly with user-made liveries actually appearing in the car selection menu.
But most of all, I'd like DR3 to become a sustainable long-term platform that will be updated and expanded for many years (like Microsoft Flight Simulator, for example) instead of throwing it all away and making a new game every few years.