What I feel should be done before Squad decides to develop any other Kerbal-themed games would be that they create a little more background on the Kerbals. What I've wondered about the most are the following:
Cities don't exist on Kerbin because of the hit they would take on performance, but if they did exist, how would they look?
How would Kerbin's landscape really look? Currently, we only see low-res, repetitive ground scatter and a homogeneous-textured surface for terrain.
How do animals look? I remember seeing a kraken-like creature in one of the official animations; how about showing more species?