I CAME UP WITH 2 THINGS! :D
1. There has to be a way to change the colour of the interiors after buying the cars, for example the Spyker Aileron Spyder looks great with a black interior (as seen in the DLC mission). Maybe an own workshop for that? Maybe there could be a way to paint them in your own colours as well? In TDU1 this could be done in the regular paint shops, which leads me to my second point...
2. Make factory colours available at the sticker shop! Sometimes you just can't help noticing that your car is the wrong colour after buying it *cough-casino cars-cough*. But you can't repaint it in the factory colours! You just gotta do your own replica and that is kinda dumb. For example, I loved buying classic cars in TDU1 and they sometimes were in odd colours and if that happened, I repainted them with the FACTORY COLOURS, not my own replicas!