This is my idea. To go along with the VAB and SPH, a smaller facility for the construction of ground vehicles. While the SPH does fulfill this function, I think a much smaller scale and possibly a different control setup (like the difference between the VAB and SPH scenes) would help. This also allows for another set of building upgrades. One possibility would be to upgrade to "Proving Grounds" where the Rover, or what have you, could be tested on different terrain. I know this sounds a little like Kerbal Konstructs, but I am not suggesting anything nearly as ambitious. It would be a way to get Kerbals to EVA's on different parts of Kerbin, without requiring a Rocket or Space plane.