Hello, a few things that I think would really improve the utility of diplomacy in the game:

• ask/demand empires to declare war on other empires
• ask/demand empires to make peace with other empires (especially when that other empire is your ally)
• for both of the above, you should be able to threaten war if they don't comply (e.g. make peace with my friend or else you will be at war with both of us)
• speaking of which, have threaten war be a trade item - if they don't agree, you're at war. You should also be able to threaten war if they don't agree with certain treaties - e.g. open ports.
• all these actions should cost influence of course.

It would be great if AI empires made use of some of these as well, but I'm okay with it being player-only if that makes it more likely for it to be added to the game.

Also it would add ways to declare war without getting the "You attacked us without cause!" reputation penalty. Though extorting other empires through threat of force should have it's own reputation penalty.

If the above is only implemented for players, it could be an option to have a considerable relationship penalty with the involved parties so there is some additional cost (besides influence) involved and so that the player cannot just go around and buy victory with Influence but only use it for important situations or face the wrath of others as being a diplomacy bully

Well, threatening another empire would probably piss them off.