They won't have real licensed cars any time soon. GTA is one of the best selling games on the planet and that's always been without licensed cars so there's no logical reason to start now, especially since it would cost so much money.
And as far as only fast and fun cars, that won't happen either. GTA is all about making a realistic free roam world. When you go outside in real life, it's not full of fast supercars. The game needs balance and realism, that's what makes it so good.
Yea you are right. When i seen vidoes on gta4 there license plate just says liberty city. Besides if you steal a car with a license the cops would mostly be after you.