That should be a total conversation mod.
Ideas for fire nation:
Fire infantry(14 strength, 25% bonus vs. Melee)
Ironclad(18 ranged strength,18 strength)
Landships (20 strength, 33% penalty vs cities)
Airship(14 ranged strength,10strength)
Air nomads:
Bison Rider(4 movement, 18 strength)
Water tribes:
Snow warrior: 25% bonus in snow and ice.