Two suggestions from myself, knowing that I predominantly play the Forza Motorsport games:
- More online features. Example: Forza 4 added a car club infrastructure that allowed sharing of things like cars and hot lap times. Expand that to let us run full-blown leagues, where the commissioner schedules all the races, qualifying, decides race scoring, establishes event car restrictions, etc.
- More expansive user-generated content. Example: My favorite feature of Forza 4 is _easily_ the livery editor. What if the game let us make our own tracks to hold events on? What if the game let us design our own supercars to drive and paint?