With your list of examples, I agree that Chemical Warfare and Fly my pretties would work as autocast. Mustard Gas? Have you even read its description? You are going to annihilate your own army.
The situational abilities abilities like Beetles, Charm and Cripple would work best with a autocast toggle options. This way, if you have battles going on on several fronts at the rts map, you can toggle autocast on/off depending where your attention is.
