@Erremnart In a way, there already exists a "guideline" that determines what is possible regarding inter-faction diplomacy. It's just that the devs are not willing to spend a minimum of effort in maintaining it. And the "guidelines" should be the rephacks themselves. If two factions are replocked with each other in "fixed" or "no more than" mode that should be a clear signal to the players that this is a crucial gameplay / Story relationship that currently can't be changed by any in-game or RP means.
But of course, maintaining this stuff would need the devs to actually get their s*** together and explain what each rephack actually means and why it has been set that way.
