Hmm well probably more gun types would be nice but thats a given.
There are lots of ideas I can think of but I think if I would try to expand on the game, I would focus on the wheel part and try to add more interaction with it rather then with the more traditional "stat upgrade route". (Though some gun stat upgrades would be nice)
For example some wheels could even be together like cogs and spin together. Or you could add a few different wheels in few different places, though managing them would probably be harder. Maybe a few different maps too.