Do you remember the old system to heroes?
When you need to train a troop, then go to hero mansion, then retrain that troop to become a hero? He start with double atributes from your earlier version but can become more powerful with experience.
How about create this , aside a hero system?
The champion will not gain Xp like heroes. but they have health like heroes and can die.
Their level will be determined from barracks/stable or blacksmith ( max lv20)
Only one champion can be used at time.
Same crop cost of hero
no off/def bonus
predefined health recovery/day
no item usage ( or get a specific consumable )
hero weapons will give double bonus to their atk/def