Although there is very good customization right now in the game, a more advanced car customizer would be great.
POSSIBLE IDEAS-
ENGINE/TRANSMISSION SWAPS: pretty self explanatory, being able to swap out the engine or transmission for something more powerful would be great
RIDE HEIGHT: being able to make a car lower than stock or higher than stock could unlock more possibilities for the car, like being able to covert the Chevy Impala into a dedicated lowrider, or taking a BMW and making it a rally car. This feature would allow the player to modify how high or low they want their car, this would obviously affect performance in some areas.
REMOVING PARTS: sometimes you just want to make a Honda Civic into a deathbox, so you would remove any “dead weight”. This feature would allow you to remove most of the cosmetic/body parts of a car, like doors, seats, headlight, trunks, interior features, bumpers, exc. This would affect you performance, like in this case, a lighter car=faster car, but it would do extremely poorly in collisions.
ADDING PARTS: sometimes you want to make a rally car from a Honda Civic, so you wanna slap on some overhead lights, some bars for your windows, and even a bullbar if your feeling daring. This feature would allow the player to attach all sorts of items to their car, giving it a more unique look and maybe even making it better and racing or the derby. This would affect your performance, like in this case, it might make the car slower due to added weight, but the handling off-road might increase and the car might hold up longer in a derby.
CAR KITS: this would range all the way from being able to convert a normal car into a pre-made racing car, or a ready-to-use police interceptor. In the simplest form this feature would be a list of pre-made setups for that car, being able to suit multiple uses
Those were all of my ideas, I hope these were entertaining and maybe the feature of Advanced Car Customization gets added, have a great day!