I like some of the ideas in this forum about distress calls and pirate attacks. Just to add a different idea I think it would be great if some planets you land on you find the world at war. Maybe there is an apex rebellion going on and you get to choose which side to help. To accomplish this you would need to use you manipulator to build and supply resources. Basically this would give you a purpose as a protectorate and it would give you a reason to build on planets over and over again. You would still have a reason to explore to find other planets in turmoil.