Two modes....production cars and racing cars.
Production cars follows a similar pattern to GT5, low performance to hyper car in all sorts of disciplines, allowed to tune.
Racing cars goes from karts, up to touring cars (Clio cup, world touring cars, dtm, V8s), through GTs, onto prototypes, through single seaters (formula ford/renault, F3/F2, GP3/GP2, Indy car, F1), NASCAR, Rally, Drag, Drift. Ie following how a racing driver becomes a racing driver.
Also....
More types of racing, proper time trials, sprints (..Gumball style?), real road racing (eg stelvio pass, isle of man tt), hill climbs (goodwood etc), drag racing....etc
And...
Custom events/series where you chose all the details including laps, course, grid/rolling start, qualifying, your opponents (could be cars in your garage) prize money is worked out by race distance and opponent difficulty.
Maybe....
Create your own team...race livery, custom suit and helmet? Buy team members
Also....
A feature a bit like Fifa where you start on a training pitch....you can select your home/a random track and either your favourite/a random car and when the game has loaded you're already in your car in your pit garage at your favourite track ready to press accelerate...press start for the gt mode menu or just go muck about on the track