I have a pretty simple (although probably already mentioned) idea, the ability to simulate simple space missions. This could work much like the way you emit light waves. You could click on an object (lets say, Earth) have an option to launch a shuttle, turning your cursor to a crosshair, and then click on another object. It could accurately simulate the time it would take to fly a craft to that object and one could speed up the process obviously using the time manipulation options already in the simulator.
This also opens up the option for one or two more steam achievements
for example:
One Small Step for Man...Send a spacecraft from Earth to the Moon.
Edit: There could also be additional parameters such as the option to have a craft return to its point of origin after visiting it's destination.
Return to SenderSend a spacecraft to a distant planet and have it return safely to Earth.