i would absolutely love a diplomacy option where you can warn the in very concrete ways.(and which the ai could use too ofc)
Something like: if you do X to Y in Z turns, i will do this or that.
If you colonize planets in my influence, i will declare war.
If you colonize planet betelgeuse 1 within the next 5 turns, i will badmouth you to the other leaders.
If you declare war on x, i will attack all your ships in x's influence (but ideally not declare war on you directly, i wanna peacekeep without giving powerfull ships to another faction.)
if you wont declare war on me for 20 turns, i will give you this or that research in 10 turns.
I will not declare war on you right now, if you give me this research and some money. (as in, if the recipient doesn't accept the trade, you automatically declare war or w/e you put up as consequence. in galciv 2 it ws always "give me money or a planet or we might declare war on you in some indeterminate time" instead of "give me money or i will attack you right now"
and ideally things like:
If you research this technology, i will declare war. (this could be coupled with espionage/counter espionage/ a bluffing element)
If you get 10 planets, i will declare war.
If your fleet gets this strong, i will declare war.
If you enter this or that space (or get within x tiles of my ships or something), i will attack you in that specific area but not declare war. (i.e. some way to set up a blockade.) (or maybe something silimar to cold war in endless space)
i would also love it if the ai took your (known) ability to produce ships and your available techs into account when deciding on war, instead of just your current fleet strenght. The position of your fleets could also be a deciding factor. This could add another diplomacy element to open borders/sharing certain knowledge: you could try to keep your fleet/production capabilities hidden tru counter espionage, baiting someone to attack you while they underestimate your strenght. or you could boast of your capabilities as a deterrent.