The AI in Minecraft sometimes can be a bit predictable so these changes can help to prevent (I'm not saying that the new AI needs to be removed or changed entirely, only that requires variability). The following changes can help:
1) Some mobs use the old AI from Alpha, Beta or early Official Release:
E.g: A Creeper start igniting while walking on circles around the objective, the Skeletons also had a similar behavior
A slime cannot swim and eventually drown in the water
A Spider that can't climb by itself at all
Some animals that don't run when they get hit
A Zombie that increase damage depending the health left
2) Other mobs have the current AI: That's it, no changes
3) A mix between the old and new AI:
E.g: An Enderman can chase you even when you are in the water
A Skeleton that have the New AI but doesn't care about burning on sunlight
A Creeper with the stationary igniting but doesn't know how to find the correct path to chase the objective ,etc.
4) And finally the mobs can jump holes depending of its abilities
I think it would be interesting to see the random combinations of AI, and could make the game even more alive
A mob's behavior makes it what it is and these would just make them different mobs. A visual change is necessary to distinguish a mob from its counterparts such as the charged creeper and spider jockeys.