As someone who enjoys single player, I think it would be fun if the AI had options to roleplay situations. For example, a hard AI that does not build trebs or bombards (only rams) allowing you to build stone walls and focus on surviving waves of attacks. You can try this now, but once they build bombards, they just knock the walls down. Another would be an AI that relies on units countered by yours, or possibly just skips blacksmith upgrades, so you get the classic computer game experience of defeating large armies. Another option might be one that cheats for extra stone and builds large fortifications you have to break. I think most single players are not interested in the games meta/strategy/build orders and just want to have fun and the current AI just totters between being too hard or too easy without having sustained satisfying action.