I feel it'll be limiting because the way I play. I like to build a car around many different instances. Be it for drifting, high speed runs, circuit racing, and so on. I sometimes like one car so much that I'd buy it multiple times to set it up differently for each case. A load tune set-up can somewhat alleviate that, though, I suppose. I'd still just love the option to have more though. With a loading set-up, you'd have to stick with the same looking car. Picky, I know, but I'd prefer to have them seperated, rather then in bulk. I'd just prefer to not be limited.
Another would be photomode, if it is implemented. Am I going to be allowed to drive and modify any car I want while in the world? If not then that's a bit bothersome if you want to open up a gallery or something(it's something I do in any game that has a photomode.) If it does let us, then why restrict it to 5 if you'd just be able to drive any car?