In theory, the X2011, the N24 GT-R, the new NASCARS, and the karts are the only real new cars. And out of those, the N24 is the only one that wasn't in the game in any way before, because the GT500 GT-Rs were RWD while this one is AWD (It's not GT500). So we have only one new car in GT5.
That's how I see it. And based on this, I would say we won't see many new cars.