I really, really like this idea. But, I'd like to add to it.
Instead of making this hero available only to certain town hall levels, make it an accomplishment. I see that creating new single player levels has not been ruled out. So, add a bunch more. Once you've passed a certain landmark of levels, you get to build this hero (for free, you've already earned it). In order to upgrade it, you keep beating the single player levels. Each time you pass a pre-determined number of levels, or maybe a certain boss level, you get an upgrade to him.
It sounds like a fantastic idea to me. I don't really care what the new hero is, as long as it's an accomplishment and not only available to the top town hall levels. But I do really like your idea for the hero, and I think it complements my idea well.
