The only problem I see is that the easiest AI is extremely passive. It won’t grow its economy past 10 villagers or so, has only a token army that lacks most upgrades (assuming it had the resources to do better in the first place) and will never attack you. I agree that some missions are very spicy but playing them in easiest would be ridiculous, at no point would there be any threat. A new level of AI between easiest and standard would be needed.
Plus it would need alterating current campaigns for that new level, such as making timers forcing you to attack (say, the enemy building a wonder) longer, decreasing the fixed enemy forces (say, Wenceslaus’ army in Genghis Khan 5)… I know it’s possible, but it would be more than just using another AI.