I don't agree, GT5 is not just about road cars, not by any stretch of the imagination, racing is the name of the game, so why not give us features to make racing cars from road cars.
Change wheel width to give us better traction, make the tire more "low-profile" like a racing car.
Edit the wheel arches to bring the whole car down without causing the wheel to rub, generally called a "wide body kit" for NFS fans.
Change the hood to include vents, so the newly added twin-turbos on my drift car don't melt my engine.
Why no vinyls ? After all that's pretty much what a livery editor is.
Racing IRL has differing liveries on cars, ESPECIALLY drifting its a part of motorsport, not JUST a part of NFS.
So please get off your NFS hating high horse and realise that's something NFS does well, if you want to make a stupid looking car then go ahead but some of us would use the NFS style car modification features to make our car look like an authentic racer / drifter, and it would be AWESOME !!
Would improve the life / playability by a million % .