I would like to make the following proposal:
The player can set a diplomatic stance concerning borders with every individual AI. This stances could be:
1: Closed borders: Nobody from the AI gets to enter your lands without a declaration of war. Pisses the AI off a fair bit.
2: Borders open for civilian units: AI units without attack power can enter, AI unit with attack power can be booted out.
3: Open borders: As the current non aggression pact