Most bang-for-the-buck hard code changes

[Lerchey]

I'm not sure if this is trivial, medium, or huge, but here goes:

Diplomacy. Right now, either you can move safely through ALL of an empires systems, or you can't move safely through ANY of an empires systems. I would like to have an option to make systems as "hostile" or something. That way, if I set up a trade alliance with the Phong, but only want them to be allowed into the first two systems (closest to them), and not go parading through the depths of my empire, I can set up space stations, mines, or whatever, and stop them.

General John