Ok, this one is a diplomatic kinda thing, but could have a big impact on how you would play the game.

Basicallly its a border/influence treaty where you would not be able to pass through another races space unless you had permission, or of course unless you broke the treaty ad went through their space without permission (thus affectinh how the race feels towards you).

An example of how this plays is in CIV4, where your troops could not enter the other races borders unless war was declared.

For GC2 this rule could be ammended so that the race could select what types of vessels could pass through its systems. i.e trade ships or other non invasive vessels.

you would also be able to demand diplomatically that other races move thier ships from your space, or they ask your permission to pass through your space on its way to destination x

what do you think?