Following the same logic as planet studio, you will be able to build your own space center to accommodate whatever you want.
The length and width of runways/taxiways will be customization similar to how fuselages work. The price of this would increase/decrease accordingly. You will have different options for runways/launchpads (For ex. dirt, dark asphalt, worn asphalt, etc.) You can also customize the numbers and lights on the runways which can give your space center some flare and what not.
Give players the ability to also add some different buildings! Even if they wouldn't serve a purpose until campaign mode comes out, we can still have an option to add them and make our sandboxes even more unique!. Varying satellite dishes, floor levels to buildings hangars, etc.
We can also have the option to slightly nudge every part of the center. (buildings, runways, roads, satellite dishes, etc.) along the Y axis, and allow users to rotate the entire place along the Z axis. (Not x or Y axis though, to keep the entire place level.) Would allow us to make longer runways, more launchpads, and then some.
(ADDED) Spawn point editor.
allow us to create our own spawn points within the vicinity of the space center. We can nudge the spawn point, see the spawn point direction and altitude. We will only be able to rotate the direction of the spawn point on a 2D axis.
If there are any additions to make this suggestion better, I'd love to hear it! So far I haven't seen much related to a customizable space center but I may be wrong.