Things such as:
-New tires for regular vehicles not designed for Badlands terrain, allowing them to navigate certain areas they normally couldn't, while being relatively expensive, and making the vehicle harder to drive/slower on regular roads (As a balancing feature)
-Fuel bladders strapped to the outside of the vehicle that act as secondary tanks, but could be prone to being pierced by weapons fire/rough terrain, filled with the fuel can item
-MOUNTABLE GPS, because your co-pilots are useless and it just always bothered me the existing SatNavs aren't mountable, perhaps add an installable bracket, have it always display current co-ordinates in the room if mounted?
-Full chassis conversions, which would work similar to the tire suggestion, but be hyper expensive in comparison, and likely negate the negatives of the tire (Along with some other features, perhaps)
It's not a particular focus of the game, but with the recent expansions to extra-domular activities, such as fishing, I'd definitely love to have an excuse to throw thousands into a vehicle that isn't, y'know, the same one everyone goes for.