Here's another few ideas:
1. Allow players to create events online, and put some of their money on the line (Whatever amount they want) which will go to the winner. Why this idea is good is first of all to prove that you're better, and also will make people want to join and play with others
2. Also online, make teams possible, I want to team up with some friends of mine and have a shared garage (Other than "sharing" the car) without the biggest limitation of modification. Actually Teams will make several things possible, starting with endurance races to actually playing more of the game together, finally.. (Say telling you which race your team mates are in)
3. Certainly and definitely GT5 is not even close to what the PS3 is capable of in terms of graphics, So one of the most important things on the wishlist is improve code base (Improve loading times), improve the graphics of the environment as in trees and mountains and whatever, has anyone ever played Forza? GT5 cries for these graphics.
4. I do appreciate the physics of the GT series, otherwise, I wouldn't be playing, but after playing real sims like GTR, you would probably want a bit more from GT.
5. Fuel reduction is pathetic, it's either a bug or a setting, but I've played a Tesla Roadster, and the "fuel" (electric car) there was going down, probably like it should, but playing other cars on long tracks or many laps, fuel doesn't go down a bit on any other car.
6. Tires need to DIE.
7. Real damage, a 30mph bump shouldn't make you restart the game because your car won't drive straight anymore. Hence why I disabled damage, although any damage is good for realism sake.
8. Apply all of the above to the AI cars!
9. While tires need to really die in terms of car damage, Normal Cars should extinct for real. I don't know what you people thought when you first got to normal cars cockpit view and saw black frame and a window?
10. More events like Lamborghini Exclusive which put your skills to the max, however, tone it down a bit, because tuning + cutting + lots of skill still mostly puts you at 1 second, maybe, advantage at the nurburgring event. When from the very start of the track your opponents have 30 seconds advantage over you + probably no limit on the PP and it's near impossible to get it right, in reality, can you imagine a gallardo doing a 7:03 lap time?! See... didn't think so.
11. Last but not least, would be nice to have Multiplayer events with an over look for competitions (With good prizes) and maybe once in a while real world competitions (in game for real cash/medals as in)
Edit: Forgot
12. A friend of mine keeps asking me to check the internet for cars that have racing modification capability (Speaking of which, how about giving it some unique name, like NFS did, Works?) and I keep coming up to a very limited list, why not allow all cars have racing modification? So this is an item on the wishlist for him
Well most of the ideas are about polish, but there are a few unique ideas up there.