I did this a long time ago, abandoning my planets with my fleet being fine to see if I could keep flying around like a pirate group to kick ass and stuff,  just to see what would happen...unfortunately I lost control of my fleet and they pretty much stayed in the gravity well until enemies came.  I had a colony ship to take a planet, so why can't a defeated player come back or keep control of their ships until its all said and done?  I think that would be very rare but interesting, like forcing players to make sure the enemy is dead and gone and not going to come back and surprise them, what do you think?