I think, if I were an actual sponsor of the game, I'd want my logo to appear a bit more often, especially since so few of the sponsors in the game are actually worth it. So I suggest allowing players to place 'stickers' in certain places on the car and actually have them show in the races, as well as their chosen sponsors. I think this might even improve the 'attractiveness' to investors/sponsors.
I also think the cars look really bland as they are (in 3D), and that having additional decorations would make them look more realistic. Basically, giving managers the ability to add 'select' car decorations in specific places, not just the sponsors you've chosen, and then have them appear in the 3D view.
I don't remember all of the sponsors, I think only a few are real companies? And I don't know if they still provide income? But I think I read somewhere that a new car model was being worked on - perhaps designing a preset number of decorations to appear on the front/rear wing, front of sidepod, back end of sidepod, etc. etc.
I know there already front/rear wing decors, but they are very small and don't appear in the 3D race anyway.