For starters every track needs to have certain race events, ie AWD Cup, FWD Cup, RWD Cup, MX-5 Cup, Porsche Cup.
You get the point!
1hr, 6hr, 12hr, 24hr Endurance Races on all suitable tracks.
This is a huge time commitment for any player. But for those of us deep into the game, these are the kind of things we'd like to sink our teeth into. Suitable rewards and credits are a given
Drag Events.
I'm not even into drag racing but this is so easy to implement and useful for testing cars and builds.
Hillclimbs and Point-to-Point Rally Stages (preferably tarmac).
Goodwood Hillclimb, Pikes Peak, Monte Carlo, Swiss Alps. More locations and events where it's you against the clock. Not every track needs to be built with multiplayer racing in mind. The popularity of Time Trials shows there's a big community out there who want this sort of content.
Free Play Area and Test Track Facility.
The Top Gear Test Track was often used online as a free play area. We've also had Complex String which was basically a test track set in the middle of nowhere.
New Garage Facilities and Car Display Areas.
I'm driving on Tsukuba right now and the paddock and pit area is wonderfully modelled. Lets us display cars in locations like this in-game. I also really want fully modelled garages to show off our cars inspired by real museums and collectors.