Easy to use (for everyone) track editor. Something similar to what is seen in Live For Speed and the built in autocross editor. It doesn't need to be very detailed. Just give us a large empty parking lot (or empty indoor warehouse, etc). Than the user can place barriers (plastic baricades, haybails, tyres, cones, etc) to create a track. Than end it with placing simple check points (finish, time traps, etc). And if you add AI in future versions, the editor would just need the option to draw the optimial racing line.
This is a dream suggestion... but it'd make the game have loads of user created content and give great depth.
I vote for this too.