I like the idea of border sensors.. Sort of like Mini spacestations.
I also like the idea someone mentioned about territory. Planets inside territory could be consitered hostile when trying to colonize in someone else territory.
I would like the ability to add ships into orbit to protect colonized planets. I think that could solve many issues.
the game play could go like this... Pass border sensors, warning is sent about being in another territory.
get to planet, ships are in orbit, when the colony ship goes to land on said planet, a pop up box happens. If an AI is trying to land, a dialoge like, We are the Torians, we would like to use this planet to help our people prosper, will you let us colonize this world?
Yes, the colony ship passes, and it is now a torian world. If No, the AI has the choice to goto war with you to land the ship.
the player on the other hand, can now start a war and blow up the ship to show force of the territory, Or let the ship move on..