Spiked bumpers, bigger gas tanks, lower fuel consumption.
Bike could have the basket from before, to add very slightly more inventory space. Motorbike could have proper saddlebags to hold even more stuff. (Maybe they'll have less storage by default, when the mods come out, and we're getting to see the upgraded versions now)
The 4x4 could have a whole-ass crate in the back, explaining it's massive storage space.
A turret would be nice, or just somewhere where the truck's passenger can stand to be able to use the guns they already have equipped.
Also, I hope dyes work on vehicles soon, because being able to paint it a new colour would let you differentiate between player vehicles more easily.