I think one interaction that would be really cool is coming across minor races that haven't achieved space flight yet. If you are the first to come across them you'd be given a whole new set of interactions with them instead of the usual. 


These could include studying them for extra science, making first contact for either war or friendship. There's a chance with attempting friendship early on that they will attack you out of fear or will grow to be a vassal or something under you rather than be independent if you keep the good relations going.


This would allow for some interesting interactions and help the universe feel a bit more alive, where instead of everyone already having space flight it feels like there's some people a bit slower and you can change how the universe grows.