I think what you're referring to here is some sort of extended "war weariness" mechanic wherein good diplomatic ties lower it and poor relations (and/or war) increase it. It's a good idea, but the main issue is that at the moment the diplomacy is a bit too limited for this kind of implementation.
Here's a specific objection to this kind of mechanic at the moment: AI gets high power score. You have lower power score, on account of not being a military-focused empire. AI relations take huge negative dive because all the AI are elitist jerks and hate anyone "weaker" than them. Your unrest increases.
An economy-based empire or research-based empire should not be penalized with unrest hits purely because their faction power (which I think is entirely based on your military strength - correct me if I'm wrong) is lower than some AI players.
Yes I do believe that the diplomatic options are too limited right now but I think this may be a great start to add some depth to the current options Also I am not proposing any game breaking unrest penalties. Something on a smaller scale (like maybe +3 faction unrest per every state of war, -3 faction unrest per non aggression pact, and -6 faction unrest per alliance).
These numbers would have to be balanced but I can understand the issues it would bring with the AI and the difference in faction power. If the total faction unrest is not game breaking (example say you get +3 unrest per state of war with 5 AI's at war with you. Total unrest would be +15, significant but not crippling) it could be workable.
Also I do love the idea of extended war weariness where faction unrest increases over periods of extended war. This could be a great way to force the player or the AI to seek peace when their war machines start grinding down to a halt.
I also see a great potential for individual faction traits like the ability to negate war weariness or a decrease of unrest with every state of war for war loving factions.
I would love to see if any of the diplomatic options can be modded, does anyone have any info on this?