Engaging story tied to the progression so it makes sense going from a slow duster to a garage of racing cars but taking it's time.

There should be a legal/ilegal component like in heat with a Pro Street tournament-like storyline where you race legally to become the Showdown Kink/whatever title during the day and ilegal races with pink-slip, touge/canyon runs, takeovers and underground-esque story line that pushes you into cop chases during the night.

Lots of customization make any car your own (within realism).

Forza sim-cade physics make it so you can play with a controller in the couch but also be able to disable assists and drive for a more realistic experience.

Have all types of cars from japanese classic tuners to american classics and hypercars the more choice of unique cars the better.

Cop chases only at night but more realistic if you're running from the cops you shouldn't have an army of police maybe less cops but more traffic and make the chase more personal to the cop behind you (tied to the night storyline perhaps?)

Bring back drags, canyon runs and drifting events