I'm sure all of these things have probably been said already:
-I want detailed visual damage as much as the next guy, but if it causes a significant sacrifice in the number of cars in the game or means the game will take significantly longer to develop, then forget it. Instead, I'd like to see a detailed "non-visible" damage system in which scrapes and collisions with other cars and solid objects affect the car's performance. The effects would be "smart" (i.e., if a player lightly backs the car into the wall at the Test Course, it would have less of an effect than if the player hit the nose of the car against the pit wall with the same force). There should be several different types of damages as well, from transmission failures, engine failures, suspension failures, aerodynamic effects, tire blow-outs, etc. Smashing the car against other objects should not only affect the straight-line speed, but the actual handling of the car as well (i.e., if a player hits the left front of the car against the wall too severely, it should throw the camber/toe setting out of whack). During pit stops the player should be able to make repairs, but the car should still suffer from handling/aerodynamic woes.
-Make money harder to come by.
After each race, the player is billed for any repairs that they did during the race, plus a fee for returning the car to "like-new" condition. The extensiveness of the repairs determines how much the repairs cost. Rare cars, special edition cars, classic/historic cars, expensive cars, and racecars should cost more to repair than normal, everyday street cars. After a single race, a player must pay any damage fees in full. If they don't have the necessary funds, they should be outlawed from buying any cars, parts, or modifications until they come up with the necessary funds. Any race purses they win during this time shall go towards paying off their repair bills. After a championship race, a player is given a repair bill notifying them of money they need to pay for repairs made during the previous race. Once at the end of the championship, the player must come up with the necessary funds to pay off any remaining damage fees they have.
Rarely award prize cars. One of the biggest flaws about the GT series is that there is no real strategy involved, relegating the game to a mere car-collecting spree that gets tiresome quickly. By awarding less prize cars, money is harder to come by, and players would develop a sense of "connection" with their cars and not kick them to the side after only a race or two. I say that the only prize cars that should be awarded are cars that can't be bought from the dealership. So if there is a $2 million race car in the Nissan dealership, the only way to obtain it is to buy it. Better be smart with your money. Also, PD should institute entrance fees for several races, especially the more difficult races.
-Instead of giving out only prize cars as awards for winning races and championsips, different awards shall also be won, such as a free turbo kit or a free entry into an event that has an entrance fee.
-In addition to damage, parts should wear out over time. The more miles that are put on a car, the more its parts should wear down. Worn down parts affect the performance of the car. These parts may be "refreshed" for a fee (the fee amount varies, depending on the part and the car it is equipped to), but after many, many miles, certain parts will inevitably fail and will need to be replaced altogether. Individual parts, such as mufflers and turbo kits, should be able to be sold, and they shall also be available for purchase at junkyards/discount shops. Parts bought here shall often be worn down and not as effective as brand new parts, but should be cheaper.
-Ability to test drive any car that the player is able to buy immediately (i.e., the player has enough money to buy the car right away) on any track. This includes brand new cars as well as used cars. I don't understand why this hasn't been in the GT series after all these years. This would cut down on all the resets and allow players to be able to test a car before they go spending wads of cash on it, just like in real life. All street cars bought from new car dealerships should come equipped with simulation tires, not sports tires. All other compound tires must be bought from the tuner shop or won as a prize in an event.
-Custom soundtracks is a must. Take it a step further and let players customize exactly which songs and the exact order of the songs they want to hear during in-game play, replay, and menu browsing. For instance, a player could choose to have one soundtrack to listen to while they are racing, a separate soundtrack that plays during replays, and another soundtrack that plays while the player is in menus. If the players wish, they should be able to choose just one soundtrack for all areas of the game, during which all songs on the soundtrack are played consecutively without interruption. In addition to being a great idea and giving players more customization and freedom to listen to what they want to listen to, it would also save PD money in that they wouldn't have to put as much focus on obtaining licenses to use artists' music.
-Ability to paint ANY and EVERY car, including racecars, special edition cars, etc. This would save players so much hassle when trying to obtain used cars and prize cars in specific colors, not to mention be able to paint cars bought from the dealership different colors than the ones offered in the dealership. Include a palette of about 50 colors, but also have a tool that lets the player mix and customize any color they want to to get that perfect color. Players should be able to place the paint on any and every part of the car body (not just paint the whole car one color). Cars should be allowed to be painted in several different styles of paint, from metallic, fluorescent, pearl, gloss, chameleon, etc.
-Better selection of wheels. The vast majority of the wheels, save for a few, in every GT game have sucked. Players should be able to choose tires with larger wheelbases than the stock wheelbase for their car, and they should be able to buy rims that are bigger than the stock wheels. Personally, it looks stupid to me to see a big car like a Plymouth Superbird on such a small wheelbase. Players should also have the option to have their wheels stick out from the sides further than stock, which would affect handling.
-Some used cars should come pre-equipped with modifications, including performance upgrades, paints, and wheels. Also, some cars should appear in the used car lot beaten up and needing repairs and parts replaced.
-Resale value should be dependent on how many miles the car has and what modifications have been done to it. Obviously, more expensive modifications should raise the resale value, and resale value should gradually diminish as more and more miles are put on the car. Cars should not immediately lose 3/4 of their value when bought off the lot. Instead, the resale value should only diminish a little bit. When you sell your car, it should go into the used car list and remain there until the next cycle of cars comes in.
-Improve the physics of the game. When doing a burnout in a powerful car from a standstill, the car should swing in a direction, rather than go perfectly straight. Likewise, when a player slams on the brakes, the wheels should lock up and the player should lose control of the car. This definitely would make it more difficult to drive the car, but would be alot more realistic. Cars should spin when clipped, either by another car or by another object. If it is determined that a player has caused several wrecks intentionally, they shall be penalized (while ppl should be penalized when they wreck someone on purpose, some lee-way has to be given because the game could determine a wreck to be done intentionally when it was an accident).
-Before the green flag flies to signal the start of the race, each player shall control his or her own car. In races with rolling starts, the player would pace the car on his/her own instead of it being controlled by the computer. If they pass any other cars and don't give the position(s) back before the green flag flies, they shall be penalized. In races with standing, on the grid starts, a player shall be penalized if he/she moves his/her car before the green flag waves. This would make timing more crucial at the start of races and, coupled with the aforementioned physics upgrades, would make getting off the line to start a race an art rather than a mundane task.
-Players should control their car while driving down pit road. They must obey the speed limit, else be penalized. Players should have a crew chief and during the race they should be able to tell their crew chief what adjustments to make to the car when they pit, including an exact amount of fuel to put into the car. The amount of adjustments and the extent of the adjustments should be limited. Players should also have the option to have their car repaired either fully, partially, or not at all while in the pits. Players should be able to specify which part(s) of the car they wish to repair, and shall be able to choose the extent of the repairs they will make on each section. No matter how much repairs and the extent of those repairs are done during the race, the car shall never return to pristine condition for that race.
-When there has been a small, isolated wreck, a "sectional" caution shall go into effect. During that sectional caution, drivers will be warned by their crew chief of danger at a particular part of the track. Though players can still drive through these sections at full speed, they must not pass anyone until the caution has been lifted. If a more serious wreck has taken place, a full course caution will go into effect and the field will pace at a limited speed behind a pace car for a lap or two. During this caution period, players are free to enter the pits and make repairs or adjustments to their cars. Players should not pass other cars during these caution periods, else they will be penalized. The caution rule shall not be in effect in every event.
-Bring back the Event Simulator from GT2, but extend upon it to allow full customization of everything. This should be available only when a player completes the game 100%, and it should be included in the arcade mode part of the game. Players would select every controllable aspect of a race, including the exact cars entered in the race, the exact modifications each car has, the number of cars entered, the track conditions, the number of laps, damage controls, caution controls, tire wear, and the difficulty of the AI. Can be done with one or two players.
Watching the Hans Stuck BMW video at the Ring got me thinking about this. 
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