If they release free DLC's then I wouldn't mind more tracks and some exterior stuff to put on the cars that would effect their performance (assuming that they don't already have it in the game), for example wings/spoilers, body kits, air dams, etc for aerodynamics. Since I am on this topic, wouldn't it be crazy if the weather effected aerodynamics? i.e. If it was humid and rainy the air isn't as dense as with dry weather. I know that wouldn't happen and it is far too much to even consider. I am just day dreaming. As far as more tracks I would like to see Cadwell Park, Tsukuba (or is this going to be in the game already?) and maybe Summit Point.
I can't really think of anything else besides the occasional "car pack" even though this game will have a ridiculous amount of cars in the first place.