Main thing is that I want the game to be longer. Once you know where to go you can run through the game super fast. Plus, longer campaign = more time trials which, although frustrating, is a lot of fun too.
Different paths to take. The first game was decently large but a lot it was for show. It would be nice to be able to go just about everywhere which would allow us a lot of different paths to get from A to B, not to mention getting us to explore and experiment to find the best path.
I'd also like them to drop the cartoon cutscenes. They're using a new engine which will make the game look even better than the first so it would be great to see them use that for cutscenes.
A race mode. I don't generally care for MP but this game I think could pull it off well.