In most 4x games, you can pretty much talk to anyone at any time without negative repercussions. Some games have built in the ability for the AI to refuse to talk to you, but mostly diplomacy is readily available and instantaneous in it's results.

I've played games (mostly board games I think) where diplomacy was more of a resource. It took a particular figure to initiate diplomacy, such as an ambassador and potentially the more grand the request the more important an envoy would be required. Diplomacy also wasn't free. The envoy used is out of commission for X amount of time (similar to a spy) and gaining audience for the envoy costs money in terms of greasing the right palms. I am also influenced by the way Solium Infernum has a very rigid diplomatic system (and a diplomatic request costs an action, which are of high value in that game).

The reason I bring this all up is maybe it'd be worth modeling in some fashion. I'd love to see a nerf on the trick of selling the same tech to every other kingdom. If you had to send an envoy and it took 3 turns for him in travel time, plus a set 100 gold in diplomatic expenses, then that'd cut down the ability to abuse diplomatic relations. Given Elemental already has some concept of bloodlines and the value of noble figures there, maybe modelling diplomacy could fit in there. Bob, nephew of the Duke of Bloodmoor, is your low level envoy capable of trade agreements, while the niece of the king is a high level envoy capable of war/peace treaties. By modelling it, it'd also mean if you wanted to be diplomacy heavy in a game then you have to actually spend resources to do it, not just work the diplomacy screens.