Here are my suggestions:
1. Be able to use standard cars in photo travel. It really doesn't make sense that only premium cars are allowed, and this is only made worse by the fact that you can see standard cars in the photo travel locations of the main menu. Oh, and standard cars don't look ugly; they look fine.
2. Name your AI & B-Spec Driver. If all your opponents have names, why not name them yourself? It could be your friends' names or your family's names, or even your rival's names. Same applies to B-Spec Driver; finding a random name is not fun at all.
3. Remove the leveling system. This is just my opinion, but I hated the leveling system from the very beginning. I've played Gran Turismo since the first game, and I can say that the leveling system is not user-friendly, especially to new players of GT5. You need to reach a certain level to do a certain race. You need to reach a certain level to buy that car. I just find that leveling system unfair. I think that all cars should be available to use, and that all races should be restricted to licenses, just like before.
4. Put some used cars in the dealership as new cars. It's no fun trying to find our favorite car after every race. Some of these cars, like the Toyota GT-ONE and the Suzuki ESCUDO, should go back to the dealersihp.
5. Be able to pick the time and weather of any track that has those features.
6. Simplify the track menu. Like, put the option of time/weather change and forward/reverse all in one track. The track selection menu seems very disorganized.
7. Allow us to set the view of the car in options. Every time I start Gran Turismo 5 and then start a race, I am always at the mirror view of the car. Please put the option to set the view of the car back.
8. Fast-Forward the B-Spec. Not all of us can sit through a boring 20-lap Tsukuba race, or a 24h Nurburgring race. Please bring the fast-forwarding back.
9. The ability to save in championship and endurance race.
10. An opportunity for DLC. Being able to download tracks and cars, that is. Though this may be introduced as a 2.0 or higher update
11. Custom BGM for Used Car Dealership, the menu before the race, when you win a race, and others that I may have missed.
12. Be able to switch music during racing. This is especially true if you end up with a song you've already listened to. Midnight Club was able to do this, why not Gran Turismo?
13. Create custom events/championships, with the latter for Arcade Mode (both single and two-player battle).
That's all I can think of for now. I'm actually new to this forum, so I hope my ideas are alright.