I hope diplomacy in Humankind will have depth and meaning to it. Fighting a war together should leave a lasting bonus to relations. Coalitions should form if an aggressive player goes for world conquest, preventing snowballing without having to give the AI unfair buffs. Late game alliances should lead to world wars as nations are forced to either join or perish.

Economic and cultural warfare should exist: joint trade sanctions to weaken a rivals economy, spreading your cultural prestige into other nations to make their population migrate to your cities. Afterall, diplomacy is just warfare without the bloodshed

Relations between nations shouldn't collapse without good reason and furthermore these reasons should change with each individual nation, a scientific ally of yours wouldn't care about you invading tribals or sabotaging scientific rival of their's. Too many times I've seen civ games were invading a single city state makes everyone suddenly hate you for thousands of years.

All of these types of mechanics should work together to make each game a unique experience diplomaticly. Out of the million possible culture combinations should also come a million diplomacy combinations to make sure each nation feels like it's own entity with its own goals and morals.