I think it would be pretty cool if we had a way to mod cars like the gti with body kits and engine upgrades,chassis and suspension. It would add a more realistic approach to stuff like autocross where you would do suspension mods to help with handling. Again this is just an idea and would be a rather complex thing to add probably, but still it would be cool.

There is such a thing already for S3 licensed users - vehicle mods: https://www.lfs.net/files/vehmods

Suspension mods are very much possible currently.

You can do:
Coilovers (change the spring rate, damping rate, rebound damping rate, height reduction)
Swaybar upgrades (stiffer front and back)
Steering geometry: Change ackerman from stock to parallel. + Change the steering angle limit.
You can also tweak alignment like camber, caster and toe.

LFS cars already have really high chassi rigidity. So no point in rigifying chassi upgrades really.