I think more scripted events such as the Mongol and Arab conquerors-esque events should be made for the AI. However these events shouldn’t happen during every game because events such as city flips and scripted armies appearing all make the game much more deterministic.
For instance (just roughly thought out example):
In every game, for AI-controlled civs, there is 50% chance the civ will remain the same. The other 50% of time, one of three things may happen
Mongolia:
1) Larger starting army, don’t accept peace with China for first 30 turns. More aggressive (aka very likely to declare war and not likely at all to declare peace) towards Japan. Better economy.
2) Slightly smaller starting army. More aggressive towards Middle Eastern civs. Spawn army by south part of Caspian Sea. Higher stability.
3) Slightly smaller starting army. Very aggressive towards Eastern European civs. Spawn army by north part of Caspian Sea. Higher stability.
Arabia:
1) Slightly smaller starting army. More aggressive against Egypt, Ethiopia, Carthage and Spain. Spawn troops and galleys across North Africa and in Spain. Better stability.
2) Slightly smaller starting army. More aggressive towards Byzantines, Greeks and Romans. Spawn troops and galleys in Levant. Better research.
3) Slightly smaller starting army. More aggressive towards Persians and Indians. Spawn troops and galleys around Persian Gulf. Better economy.
The specific civs and benefits from this idea isn’t thought out, but I was just hoping some sort of randomization system can be in place so that civs won’t be expected to behave similarly in most games.
I'd rather leave Spain as it is. Why are you so keen on seeing France and Arabia fight over Iberia? There wasn't that much conflict between them, Tours and Charles Martell are repeatedly exaggerated in popular history these days.
I wasn't thinking that because I'd prefer France fighting Arabia. I'd prefer to see independents representing the Visigothic kingdom and both France (Franks) and Arabs (Ummayads, Berbers etc...) fight them rather than an actual Spanish playable civ. I also feel that it is easier to almost guarantee Spain a successful Reconquesta of their core area if they spawn after the Arab invasion event(such as your horde spawn idea).