I'd thought of this, too - it's just not very Gran Turismo... still, Kaz did say GT5 is a different take on the idea! 💡
Editing in this manner also means that sharing the tracks is easy, since the game generates it on the local machine from just a few pointers to stick objects in certain places... Handling of the circuit itself is harder, so it must be constrained to certain sections - especially if the AI is to race there...
They are freely available to everyone, for a price

But anyway, I also think it'd be unlikely, so the only alternative is
new content via download.
I think you meant that the cars were unlocked in GT mode (you just couldn't afford them all) and you had to unlock stuff in arcade mode? Either way, what does it matter? If the car / track database in the game is designed in such a way, it'd easily accept DLC. I think the basic version and premium versions will contain an identical
game at launch, just that the premium grants you free downloads later on?