I think what should've been done, is that they should've made the prize cars non-random, just like in GT4/5/6, and that the prize car would potentially have a connection to the event you got gold on. It'd reduce the number of duplicates and it'd add some flavor, like winning a basic hatchback for some of the driving school series, or winning a Gr.3 car for completing the Nurburgring Nordschleife circuit experience. And then as more events are added, certain cars could become prize cars. For example, I would've made the prize for the Porsche Cup the 919 race car, and the Gr.4 Audi TT the prize for winning the Tourist Trophy. On a side note, I'd also love to see some race series, in which you obtained points and what-not, both off-line in GT League and online, though for all we know, that could be coming in an update somewhere...
I think winning a prize car in GT6 also let you pick the color as you won it, too, though I'm not 100% sure....