I like the idea. I think heros should get tired. Id like the game to be less city centric. Maybe for age 3 make the hero the center of attention. Say the main hero is the player lol. Since it is the player it cant be killed. A high stat hero is more valuable then a high stat city.
For the newbies:
My best attack hero is worth more then any city because i can build army fast in any barracks in any city.