Okay, played some more in another game, hopefully I can refine my feedback so it's more useful:
1. AI player seem to focus on score more than anything. In my playthrough, India had churned out a ton of wonders, which earned him universal scorn—despite that he had only 3 cities, none of which were coastal, and started no wars. Though he was technologically advanced, I guess he was "peaceful" and a non-expansionist. This meant that I (along with the Shosone to a much greater extent) were able to wage a few wars without much consequence. Eventually, I was able to eliminate India, which meant every AI, including previously friendly ones, immediately turned on me. Including AI's who were being actively being conquered by the Shosone.
2. World Congress seems... slightly imbalanced. If you vassalize a civ with a lot of WC votes, they'll nearly always propose Global Liberation, if available. It's more effective to wipe these civs out completely rather than let them capitulate, since they'll become hostile/afraid once they're freed. And then might even declare war. Despite having been conquered once already. I'd suggest that vassals must vote according to how the master civ votes, and cannot propose liberation in the WC.
3. Some WC proposals should be tweaked, or at least the AI's reasoning to propose them. Sanctions are weird, in that they're much too punishing (no diplomacy options at all, permanently) for AI civs, but for players, you can easily circumvent this by waging wars and having nearby AI's capitulate. Further, AI's that trade with you for or depend on your strategic/luxury resources will propose sanctions regardless. Sanctions/Open Door/Sphere of Influence should have a turn limit, bare minimum. I'd further suggest that sanctions be adjusted so that breaking them is possible for non-vassalized civs.
4. Again on the topic of the WC, the United Nations proposal is rarely, if ever, passed. Perhaps because it's a requisite for a diplomatic victory, but it's just unintuitive; World Religion not passing is understandable (despite not having a religious victory possible), but the UN seems to have no transparent reasoning. These two also allow for some meta-gaming strats with the WC.
5. War-mongering diplomacy penalties are both confusing and predictable at the same time: predictable in that, if there's a more hated civ (i.e., one with a greatly higher score) you can get away with conquering some other civilizations with little penalty. However, should you take out the highest scoring civilization, the other AI's will never be grateful, but instead turn against you. It turns into a game of hot-potato, essentially. Civs you give cities back to will also never be thankful, so long as your score is much higher than theirs.
6. War-mongering diplomatic penalties seem entirely based on conquering cities, and the penalties are incredibly high and take much too long to decay. When coupled with the score based system of AI opinion, this leads to bizarre scenarios where one civ will denounce you and/or go to war with you, despite another civ having taken over some their cities and potentially about to conquer them entirely. Shared religions/ideologies do very little to prevent this, either.
7. There's a meta-game tactic to prevent the penalties from conquering cities, which is allowing allied city-states to take them instead after you knock the cities health to 0 or enough to where they can conquer it. You can cripple other civs this way, and since city-states are "neutral," these same AI players won't attempt to reconquer city-state controlled cities.
8. Unintuitively, civs that are Afraid of you will not (or usually cannot because they won't establish an embassy) willingly become a vassal. I've a civ become a vassal willingly once, but that required friendly relations and trading a tech. Despite that you can obliterate them to the point where they fear you, they somehow think they have a chance at winning.
9. I know I've seen this mentioned before, but there are way too many "impossible" trades. Then, without reason, some of these impossible trades suddenly become possible. Then impossible again. Trading cities is one of these, vassal state is another. A weird quirk of this is that only AI players can propose Declarations of Friendships. Even if one expired and you try to renew it, they'll refuse until it's their turn. Other friendly civs will also avoid DoFs despite having all modifiers in the green, being Friendly, and not having had made DoFs with a denounced AI.
Basic summary, I guess, is that the AI prioritizes winning much higher than surviving. If the AI prioritized setting up a stable, secure civilization over meeting win conditions, then that would pose more of a challenge than the prioritizing victory first. Score-based victory should be heavily discounted by AI, since it ends with the AI sacrificing themselves while being entirely predictable. This doesn't mean that AI players need to be passive, but just more conservative at estimating their chances for victory, and willing to sacrifice victory if it means compromising their civilization stability. Essentially, the AI should generally prefer peaceful solutions, while being aggressive if the situation warrants it. Allowing short wars would also be useful (being able to negotiate peace without a turn minimum), since both players and AI can wage strategic wars to take over/raze a city (e.g., you use a citadel to take over a strategic resource or vital luxury such that it disrupts the AI, they should immediately declare war and attempt to capture or raze your city, then negotiate peace or play defensively if the player doesn't accept). Once you have a war score of 100 (or -100), you should be able to negotiate peace instantly.
1. AI player seem to focus on score more than anything. In my playthrough, India had churned out a ton of wonders, which earned him universal scorn—despite that he had only 3 cities, none of which were coastal, and started no wars. Though he was technologically advanced, I guess he was "peaceful" and a non-expansionist. This meant that I (along with the Shosone to a much greater extent) were able to wage a few wars without much consequence. Eventually, I was able to eliminate India, which meant every AI, including previously friendly ones, immediately turned on me. Including AI's who were being actively being conquered by the Shosone.
2. World Congress seems... slightly imbalanced. If you vassalize a civ with a lot of WC votes, they'll nearly always propose Global Liberation, if available. It's more effective to wipe these civs out completely rather than let them capitulate, since they'll become hostile/afraid once they're freed. And then might even declare war. Despite having been conquered once already. I'd suggest that vassals must vote according to how the master civ votes, and cannot propose liberation in the WC.
3. Some WC proposals should be tweaked, or at least the AI's reasoning to propose them. Sanctions are weird, in that they're much too punishing (no diplomacy options at all, permanently) for AI civs, but for players, you can easily circumvent this by waging wars and having nearby AI's capitulate. Further, AI's that trade with you for or depend on your strategic/luxury resources will propose sanctions regardless. Sanctions/Open Door/Sphere of Influence should have a turn limit, bare minimum. I'd further suggest that sanctions be adjusted so that breaking them is possible for non-vassalized civs.
4. Again on the topic of the WC, the United Nations proposal is rarely, if ever, passed. Perhaps because it's a requisite for a diplomatic victory, but it's just unintuitive; World Religion not passing is understandable (despite not having a religious victory possible), but the UN seems to have no transparent reasoning. These two also allow for some meta-gaming strats with the WC.
5. War-mongering diplomacy penalties are both confusing and predictable at the same time: predictable in that, if there's a more hated civ (i.e., one with a greatly higher score) you can get away with conquering some other civilizations with little penalty. However, should you take out the highest scoring civilization, the other AI's will never be grateful, but instead turn against you. It turns into a game of hot-potato, essentially. Civs you give cities back to will also never be thankful, so long as your score is much higher than theirs.
6. War-mongering diplomatic penalties seem entirely based on conquering cities, and the penalties are incredibly high and take much too long to decay. When coupled with the score based system of AI opinion, this leads to bizarre scenarios where one civ will denounce you and/or go to war with you, despite another civ having taken over some their cities and potentially about to conquer them entirely. Shared religions/ideologies do very little to prevent this, either.
7. There's a meta-game tactic to prevent the penalties from conquering cities, which is allowing allied city-states to take them instead after you knock the cities health to 0 or enough to where they can conquer it. You can cripple other civs this way, and since city-states are "neutral," these same AI players won't attempt to reconquer city-state controlled cities.
8. Unintuitively, civs that are Afraid of you will not (or usually cannot because they won't establish an embassy) willingly become a vassal. I've a civ become a vassal willingly once, but that required friendly relations and trading a tech. Despite that you can obliterate them to the point where they fear you, they somehow think they have a chance at winning.
9. I know I've seen this mentioned before, but there are way too many "impossible" trades. Then, without reason, some of these impossible trades suddenly become possible. Then impossible again. Trading cities is one of these, vassal state is another. A weird quirk of this is that only AI players can propose Declarations of Friendships. Even if one expired and you try to renew it, they'll refuse until it's their turn. Other friendly civs will also avoid DoFs despite having all modifiers in the green, being Friendly, and not having had made DoFs with a denounced AI.
Basic summary, I guess, is that the AI prioritizes winning much higher than surviving. If the AI prioritized setting up a stable, secure civilization over meeting win conditions, then that would pose more of a challenge than the prioritizing victory first. Score-based victory should be heavily discounted by AI, since it ends with the AI sacrificing themselves while being entirely predictable. This doesn't mean that AI players need to be passive, but just more conservative at estimating their chances for victory, and willing to sacrifice victory if it means compromising their civilization stability. Essentially, the AI should generally prefer peaceful solutions, while being aggressive if the situation warrants it. Allowing short wars would also be useful (being able to negotiate peace without a turn minimum), since both players and AI can wage strategic wars to take over/raze a city (e.g., you use a citadel to take over a strategic resource or vital luxury such that it disrupts the AI, they should immediately declare war and attempt to capture or raze your city, then negotiate peace or play defensively if the player doesn't accept). Once you have a war score of 100 (or -100), you should be able to negotiate peace instantly.
