This may need its own thread, but since you are already working with relations and diplomacy... I thought it would be good to run this by you first:
I think it would add a lot to be able to make threats in diplomacy. The way this would work would be like... I'm at peace with my neighbor and I want them to cancel deals with my enemy who I'm at war with. It may be even be red because they likely do not want to cancel deals. Having a new option, I could put cancel deals, and click threaten war. Several things could happen... If they calculate I'm weaker then them, they may declare war. If we are equals, there could just be a high negative similar to making demands but higher. If they do not want war they would do what I wanted and cancel deals.
Threats are an 'in-between' option of just declaring war over something like open borders or making demands that go nowhere. It would be nice to make a demand, they say no, threaten war, and they give in... or war is declared. - And it would be nice to make threats over red options that they would really give in if it was war.