And how does that work with planets? Do their planets also become immune from attack? Are they able to steal planets with no repercussion?

Also, can someone go on a huge pvp rampage, steal some planets (annex decree, paradox generator) and then switch to diplomacy mode?

Yeah maybe there should be more interaction with other legions in the game to create war or something. Maybe an occasional reward drop to the winning team.

Just a suggestion that probably wont work. Might make the game laggy as well. How about a legion mission with a live chess board of ships from each participating legion that need to do something , like get to the other side and capture something? Ships can lock and attack a nearby enemy ship and disable them. Guess there should be like a one minute timer to be able to move around per square or run away maybe warp out and retreat from battle as well and forfeit the legion mission. The legions should be matched by average strength? Just putting some sorta idea out there.
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Or

Could also be a mini base where ships have to defend a base for a certain number of hours while other ships try to whack the opposing base. Ships can attack back. Of course no halcyons or calming amps. Just like defending a planet, except its a base. Ships defending defend, while ships attacking attack the opposing defending ships guarding and the base. But defending ships can switch to attack the base and vice versa.