I was posting on another thread talking about ZoI vs. ZoC in this game and had an idea that I think would be interesting for how the AI interacts with the player.
You no longer need to declare war to to attack a ship or starbase. If you attack without declaring war you create a diplomatic incident and are contacted by the attacked player. They confront you and you can respond in a few ways:
1. That was the first salvo, it is time for war!
2. I'm sorry it must have been a mix-up, my troops were conducting training exercises in the area and your ship must have been caught in the crossfire.
3. My fleet goes where it likes and does what it likes, declare war on me if you dare, but I will burn your puny civilization to the ground!
additionally, if the incident was within your ZoI, you get the option:
4. I will defend my borders with force. Enter at your own peril!
Depending on your diplomatic relations and relative power, the AI might declare war, or might say, "we'll forgive you this time, but don't let it happen again," or "Yes, we will avoid crossing you in the future, just please don't hurt us!" or something else. If you choose option #4, it will let all AI know that you intend to destroy their ships if they enter your ZoI. Creating a diplomatic incident will in most cases hurt your standing with all civs.
To me, this allows you the chance to try to enforce your ZoI or not, depending on your preference; and makes for much more interesting interactions than simply not allowing you to defend your territory (or try to claim someone else's) without an outright war.