As something I thought of recently, I mentioned a showroom you can actually navigate. Some people don't like the idea because it takes up too much space. So here is a 2D solutiion:
Instead of a top-down view of the showroom, a generic showroom shows filled with all sorts of cars. Instead of the cars in their top-down view, the cars are replaced with the icons of them. So for example, Honda has maybe 10, 12 cars in GT3. So all the machines up for sale or for viewing will be displayed by their icons. And when you choose which car you want to look at, you will get the showroom view of it, as it were on display in New York, Detroit, Geneva, or whatever. I think that the cars displayed should get a nicer shine job. Like maybe you can have the big lights over the cars, but have maybe the cars surrounded by a starry background. To me, this will give the car a sparkly, shiny effect once viewed. Custom backgrounds and effects for each car, as I mentioned, will also be special. I think that a navigational showroom more than the list of cars will stay true to my "3D Showroom," but will not in any way slow down or frustrate shopping.
Another consideration I have is that if there is a seperate tuning division (for example, AMG for Mercedes-Benz, Spoon Sports for Honda, etc...), there should be a seperate "showroom" featuring those machines. This next one might be a good idea. If there is a car you unlocked by winning races or a championship, you have a chance to "unlock" that showroom. For example, if you get a Mini Cooper in GT3, my GT4 example will allow you to access Mini and maybe buy cars in the color you so choose. You can even get race cars and concepts, but as in an old post, you'll need maybe an International License depending on its power and capabilities, and a Special License for the concepts. Then if you sell a car that cannot be sold at a car dealer normally, you may be able to buy the car again since you've unlocked it. I think this will avoid the trouble of getting an earnable car and then selling it. Because when you sell that machine, it's gone. At least you can be able to buy new ones, pending you have gotten a car that cannot be bought. The only exception to the rule MIGHT be the Formula One cars. Those cars kind of look like real-world machines, but with generic paint. Then again, name me one car company where you can buy their F1 car right off the lot. The F1 Machines are a definite "must get an 'S' license to race."
This last one concerns used cars. First off, I think all companies in the game can use a used car lot except if the lineup of cars isn't very big (for example, the Lister Storm in GT2). But if you sell a car on one day, you may be able to buy the car again for a cheaper price before the car and its tuning is sold to another person. And of course, the more tuning to a car, the more you can get back in return. So if you have a 25,000 Cr machine and tuned it for over 200,000 Cr, the formula for the price will most likely be:
(car value/4) + (price of tuning/2) = new sale price
I'd also boost the sale price based on the car's mileage. So a car that raced a 150-mile endurance can sell for higher, but can be bought back for less. And with the selling option, you keep everything you've modified to the car. But if you seriously want to sell a car, when you sell, a dialog will read: "Do you wish to sell? NOTE: if you seriously want to sell this machine, you cannot buy it back in its form at all."
Well, three more ideas, and it looks like I'm a "Bronze Member" now... I'm shooting for gold, you know that.