As we all know nowadays, most cars are the same underneath. In the game will you be able to buy parts, such as chassis, engines and body shapes from other companies? Would you also be able to sell yours as well?
Another idea i had was an online auction house for all of these parts. You could integrate AI parts with real user parts in one big online auction house.
I had this idea a while back on chat, if you remember it 
jack.
Though still something to look into:D (IMO)