Keep in mind that these are all just suggestions and do not have to be implemented in GMR. I understand that this is a lot to read so feel free to skip around and look at what interests you.
--Parts wouldn't have to be sold and bought again for different maps. Any parts that you have bought can be equipped and unequipped depending on what you want for that map.
--Manual Transmission and Semi-Automatic Transmission options. I know it can't actually be done because of the way the source engine works but I know a way to simulate it: every gear change the vehicles top speed and horsepower would change for example:
---A car with a top speed of 65mph and 500hp
1st TS (top speed)-15 mph HP (horsepower)-600
2nd TS-25 mph HP-500
3rd TS-35 mph HP-400
4th TS-45 mph HP-300
5th TS-55 mph HP-200
6th TS-65 mph HP-100
--An XP and leveling system that would replace prestige. Instead of having to buy all the upgrades to prestige, why not have an XP bar? Once the bar is full you would have the option to prestige or level up even if you didn't have all the upgrades. The higher you place in a race the more XP you would get. There would also be bonuses for racing with a manual transmission or beating someone's record.
--Two types of engine upgrades:
1. Size increase starting from a one liter engine and increasing by 0.1 liters per size upgrade up to ten liters. However the size of the engine in liters can't be greater than the amount of cylinders for example if you had a V6 it couldn't be bigger than six liters. Each would give you +5 horsepower and +1 mph top speed.
----Cost System: stock 1 liter
upgrade 1.1 liter $100
upgrade 1.2 liter $100
upgrade 1.3 liter $100...
...upgrade 2.0 liter $200
upgrade 2.1 liter $200
upgrade 2.2 liter $200...
...upgrade 3.0 liter $300...
...upgrade 4.0 liter $400...and so on...
2. Cylinder increase starting with two cylinders and going up to sixteen cylinders. The purpose of adding cylinders wouldn't be so much to increase speed or horse power as it would be to increase the maximum size that the engine can be as mentioned earlier. It would also change the sound the engine makes as it always has.
----Cost System: stock 2 cylinder
upgrade 3 cylinder inline
upgrade 4 cylinder compact car engine
upgrade 5 cylinder inline
upgrade 6 cylinder car engine
upgrade 8 cylinder muscle car engine
upgrade 10 cylinder sports car engine
upgrade 12 cylinder super car engine
upgrade 16 cylinder hyper car engine
--Sliders for gear ratios, armor to weight reduction, spoiler downforce to drag, and turbo duration to turbo cooldown or turbo force and speed to turbo duration. Also there should be switches for anti-lock brakes, traction control, and stability control which if switched off would decrease roll speed and handling but increase XP.
Again these are only suggestions although I do hope at least some of them will be added in some time in the future.