Sound mods, car wheel mods, car colour mods (orange enzo with black rims, clk-dtm black with black wheels with a silver rim). Looking at the images above looks like some are working on the ability to convert the actual car models themselves. TDU's future will be prolonged with this and could even bring more people to the game after they see what may be possible.
Remember Atari Germany's stance is whereas they don't mind you posting mods and such, they warn that they will not be responsible for anything that may happen to your game or profile using them and though they don't say 'yes we approve', they don't exactly mind it being there but they aren't looking at it while it's going on, keeping a distance so to speak.
In other words, Atari Germany may be willing to discuss it on their forums, but other regions don't.