2 simple things that would provide a lot more options in the game are:
Braking Inputs. Resizeable Wheels should have an input for braking, similar to how engines have an input for throttle. This would allow for handbrakes and fake burnouts and just all sorts of cool stuff.
Improved Shocks. Imagine you want to make a really cool crawler with some nice shock absorbers, with really thick rebound damping to control the vehicle but with barely any compression damping to really soak up those bumps. Well find a different game because SP doesn't have that (yet)! Personally, I think it would give a ton more options for your suspension, such as a monster truck type suspension with really bouncy rebound.
That's all for my list so far. I've heard a lot of ideas of huge things to add to the game, but these were just 2 small things that I think would be pretty nice.