I have seen the comments and to clarify, each planet would have a unique valuable resource, (e.g. methane, dry ice, gold) this is just some examples of what some may be. there would also be the aspect of the unknown with players being able to be the first to step foot on a new planet with no map, players will be able to be the first to map out there planets and explore into the unknown, one thing would be an online map of each planet only being made public after a certain amount of players, nations have set up outposts there, another thing(if possible) is making it so each nation can only have one or two towns in space (perhaps in a space station) and nationless people not being allowed a town, there by making sure earth isn't abandoned. ships (or at least long distance ships) would also have to be made out of valuable materials to make space travel a late game mechanic and to add space piracy. Tre_2020 pointed out the idea of a siege region for places like the moon so I think that that would be good for each nation to fight over the moon and or other planets.