It would be really interesting if some of the advances in AI could make their way into the game. Make the gameplay more human by incorporating recent advances in ML-based gameplay e.g. Diplomacy and Poker. Create more variety of dialogue using transformer models. Make in-game diplomacy more nuanced. There’s a lot that could be done.
The dirty secret is we think we want the AI to play more realistically, but actually don't. This is true in many games but especially Civilization, where we rely on the AI behaving predictably to feel mastery over the environment, and for diplomacy to feel "fair". If the AI were playing to win, diplomacy would mostly evaporate as a game system. It'd break the fiction of the game.
In SMAC you could make a single proposal per turn. If it was good, they would accept it, if it was almost good, they would counter your offer, if it wasn't good enough, they would actually get madder at you and stop talking for the rest of the turn (or for a few turns if you relations were that bad). That was the only feedback you'd get.
This small change made parlaying with AI leaders feel more realistic. Instead of going "Hm, you won't give me Zaragoza and all your money, because 'we are losing' is only at +15, I'll come next turn when it should be at +17" you start building a mental model of each leader and proposing deals you think they should accept.