I am currently working on some things myself to do a large suggestions regarding player Car shops, Competition. Between them and what each should do to make it a viable income.
I will give an insight on the illegal mods side.
Nitrous: I haven’t got as far as this stage yet but the idea behind was to give the vehicle a 1.25 / 1.5x drive force boost (Vehicle based off the handling file that controls engine power, means all cars will be different) will give you a boost for up to 5 seconds however, needs to be timed properly otherwise you run the risk of blowing your engine. Example you are at the cars top speed of say 200mph and you hit nos. You are then hard red lining the car and therefore damaging the vehicle causing the engine to blow up and disable it. Use it more tactically exiting a corner onto a long straight for example to give the boost up to the top end.
Performance Limiter removal. Idea is that all vehicles in RP are limited to 160-180 dependant on vehicle. Better for networking to cause less ghost cars, Desnyc locals being invisible Etc. The performance limited remover will improve top speed by around 20mph and possible acceleration.
Both will be purchasable through player(s) but have to be found in rp and only installed on vehicles the seller trusts as they could be ratted out to cops and arrested for doing so. Again the idea I like of build relationships and rp to get certain things.
If caught using the mods vehicle can be towed for maybe a week where the mods will be removed and you will have to pay to get the car back. Note not something you should have on a daily car should be rare used for racing etc.
With the engine, transmission turbo suspension etc. I had the idea of making the final stage 3/4 only buyable through tuner shops owned by players. The modifier could be increased slightly make it slightly more rare and more difficult to get. I also have the idea of having a remap system which I am looking to work on in the future that you can choose to go for top speed or acceleration or find a good spot in between. Just an idea not looked into it yet so may not be viable.
suspension is a difficult one as going any lower then what is usually the setting 4 in ls customs tends to cause ‘scuff’ where your wheels will go through the arches of the car.