Yeah there's going to be hero characters that are actual fleshed out characters. But I am going to try and make the story change depending on who dies. So it's a little more important to keep cheap ally deaths down, so that you don't permanently lose the character and have to start over your save file.

That causes problems. No one will be likely to take the risk to loose their character and give it to AI. I still think that there could be some unimportant allies that could be controlled with less care- it could increase your fighting freedom and still don't take that much care. Your hero would be the main character, but you could do riskier things with other troops.

If you make different story depending on who dies, I think you couldn't have more than two main characters as AI would need to chose which of the characters dies when you're controlling just one.

You will have to rely on different classes than I think. If a player tiers of playing with sword he can pick up an archer.

They are going to be different classes of characters. I do like the threat asessment idea though, that sounds like it would keep them alive longer. How do they do it in Brutal Legend?