If it requires adding a new car manufacturer, I doubt we'll see it this time around because they would need to negotiate the licensing for it. But I wouldn't think it would be too difficult to add some vehicles from the manufacturers that have already been included in the game?
I know it may be silly, but I think a premium version of the Veyron, or to more properly go along with the OP, the Veyron SS model. I mean, the Veyron wasn't around for GT4, so it makes no sense why it ended up standard? I'm assuming of course that a healthy percentage of the standard cars are simply "ported over" from previous versions. And it is or at least was the fastest car on the planet, and it IS totally different than a McLaren. I think it deserves more respect from the "greatest car game ever" than simply being modeled hastily.
For crying out loud, have you seen the spoiler on the in car game? It's animation is horrid!
Maybe some other "exclusives" like an FXX or something to that end?