I've had a read through the suggestions and found one fairly close to this idea (SOL stations being collaboratively built), but I wondered if it would be a good idea for the effect of the Bulletin Board missions to be felt, so as colonists and prospectors are transported out to various places then colonies will form which will slowly build up to form new completion/origin points for missions.

E.G.  Each planet has a Colony level and each level relies on a certain amount of goods to be delivered specifically via a mission from Earth (due to the random nature of missions this should hopefully make colony progression fairly slow).

As colonies build up they will in turn offer more profitable "return to SOL", System or Interstellar missions.

You could also have other mission types, such as "defend colony" (for the combat orientated players), "survey site" (to establish new colony points on other planets/systems) or "transport goods" (to upgrade existing colonies).