Actually, if you borrow my idea of special abilties in items, you might be able to make one big modular AI with a small modules attached to each new skills. They could change the weight of other behaviors like when to flee, when to charge, when to stay at distance, add new triggers and of course their own weighted action to when to use that particular skill.If me and Luma have to implement all this...
If you can make it so they don't look completely dumb, only a few handful of active and passive skills would be enough to create thousands of combinatory slightly different AI. Just gives some random items to your orcs and you will be good to go.
You'll get a prototype mod with thousands of different AI. From there you could progressively add new skills and expand on it.