Here is my 2 cents worth:
1. I want to have a say which cars I want to race in the regular gameplay. If it's a classic, American or Italian race (for example), then I want to amend the list of pre-selected cars and choose which cars I want to race. It's that sometimes, I feel that some cars just don't belong together. It could be because of its period or region. I love the Photomode, but hate when taking a classic black and white photo with a modern car in the background. Its such a lust-killer.
2. I just love looking at my cars in the garage, you know the feeling, it's like you actually own them. The trouble is that having this division of standard or premium cars makes my searches a hassle. For example, I want to see all my BMW's, but I can't see them all at the same time because of this "division".
3. Instead of looking for your cars in the garage through thumbnail pics, why not have them presented side-by-side in a 3d garage, cycle through them with at least 3 in full view at a time and if you like, zoom to the one of your choice in the center.
4. I want to classify and "file" into my personal folders. Better yet, custom tag my cars, such as, tuners, stock, GT, roadsters, NASCAR, hyper-cars, 70's, 80's, DTM, drift, F1, rally, show, one-offs, prototypes...you get my drift. So if I am in the mood for a thrashing in Le Mans, I'll search my garage for my tag "LMP", a little classic Monte Carlo then I'd search for "60's". I don't want to see all those irrelevant cars. Maybe, even have multiple tags, "60's"+"LMP"+"European".
5. That reminds me, how about mix class racing. LMP/GT, like the real thing. You are driving an LMP and have those pesky, slow GTs in your way or you are in a GT but have to keep looking at your 6 to avoid an LMP being crammed up your tailpipe.
6. I hate the fact that there are a million MX-5/Miatas. I understand, it was a question of manpower and PD needed to bump the number of cars for bragging rights, but, ultimately it's wasted storage space in the disk. I rather have 500 "unique" premium cars than 800 cars; 300 of those "clones".