Ah, you know how it is. A neighbor loses a planet to a common enemy and you swoop in and take it. Except the original owner (whom you have no wish to go to war with) still has a starbase and/or a shipyard in your new system. A chance to trade for or purchase the bases and shipyards would be great.
A feature that may be cool in such an event as mentioned above is the concept of "Liberation", i.e., capturing a planet and having a choice for ideology points to either return it to the founding civilization, offer it back for a price or ruthless annexation.
Luceo Non Uro