A few airports and launch/landing pads dotted around the planet, they would be able to refuel your craft if stationary within a certain distance. Repair could be an option too. These places could also have missions, carry X amount of cargo to port B for example (Also send this module into X orbit.). This would provide a means of cash and would add purpose to craft costs, as well as giving planes a purpose as well. Getting there in a short amount of time nets more reward, so will you take a plane for efficient cost or a rocket for faster travel?
(This may seem like two ideas, but missions would only work if there were more places.)
They will add a carrer mode!