The issue with open borders is so glaring that it destroys any sense of immersion for me. I do not expect this to be officially fixed anytime soon, as it should have been fixed during development (not a good sign). So I was wondering if it would be possible to mod the game's core rules to solve this issue?
Either adding a restriction to require a declaration of war before entering the borders of another civ, and/or allowing a ruling civ to attack ships within it's own borders without a declaration of war. The second option is one I've never seen in a 4X game and seems more accurate to real life diplomacy "You can't blame us for destroying your ship, it appeared suddenly out of warp right next to our planet, and we are at war with another race so our military is on high alert on that world."
Is a mod of this nature possible?