My idea is that once you max the troops Barbarian, Archer, Goblin, Giant, or Wizard, you make a choice if you want your troop to have extra defense or extra offense. Once you make the choice the troop gets extra features added.
Examples: Barbarian (Defense) adds black shield to match his helmet and sword, or (Offense) adds a black great sword. Defense + 30 health, Offense plus 10 DPS.
I have more examples but I don't think anyone wants to read that much. This concludes my first suggestion.
