One car I wish were paintable (apart from all the cars mentioned by you all) is the formula gt. I know it's a race car, but it isn't real, so the liveries are all make believe, why the hell isn't there a paintable base model for that???
What I would like is for cars like that, which have basic multi coloured liveries that aren't real, is to be able to paint each colour seperately. Even with the two basic livery patterns on the fgt, you could paint them to make them very close to f1 liveries. Or, with the huge number of different paints available, there would be an enormous number of custom liveries for race series'.
It would also be nice to be able to do this with road cars that come with stripes and two tone paint jobs from the factory. Instead, some cars you select without the stripes, then paint them and stripes appear, or vice versa. It's very annoying, and really wouldn't be hard to fix.