An idea I just came up with was that if you have a grassroots race car in GT (and that's how sane people like to start out), normally, a few decals on the side would be good. The race cars that have some of the ugliest use of sponsorship decals (no offense to such fans) are either NASCAR or some street drag race cars. Being the owner of three NASCAR PC games, I'm used to pasting several on-the-side decals. You know, the decals like EA Sports, Moog, Gatorade, and all those.
Now, I seen import and sport compact cars (I'm not calling them "ricers") with side decals like Greddy, BF Goodrich, Motegi Racing, and all. So if you could tack on some of these sponsors to a street car you want to mold into a pure race car and don't have a lot of money, just buy a pack of decals for your car. If GT4 wants to be more realistic, you would have to try and EARN sponsorship by completing special races, like in Sega GT, even though I haven't played all of it on PC.
Now, an interesting consideration would be to get sponsorship from past cars. But I don't know if a person would want to paint up a Supra with Autobacs or Castrol sponsorship without sponsorship consideration. Maybe we can give PD this consideration. I like this topic idea. Then maybe gir racers might go for the tuner club colors of Yashio Factory, hot pink colors. So customization would be a hell of an idea, especially for online play. So you get to play people with your own personally-customizable GT car. I apologize for not simplifying my topic, but I had a lot to say about it.