Unless the car(s) in question are real, standout hero or landmark classics for a brand, it wouldn't make a whole lot of commercial sense for Kunos to go chasing and spending a fortune acquiring a niche historic license I'm afraid. Rememberm, they've put a lot of their own cash into the game, so it needs to make a return for them or we can say bye-bye to ongoing support for the sim.
I'd like any new licenses to be strategic to be honest i.e. ones that will expand the game's appeal and exposure around the world. As an example, would a Chevy license gain the sim more attention and users in North and South America? Or would Toyota or Nissan get it some real traction in Japan and the Far East? I'd like to see that. Same with new track licenses - a good geographical spread of interesting tracks would unlock the world for AC I think. I suspect it may be being seen as a bit 'Euro-centric' at the minute. All depends on the funds available to Kunos at the time of course, I know that