I've been thinking that it would be a great idea to upgrade weapons and not just troops.
For example,

Wall upgrade: Outer Perimeter could be loaded with troops on top that run around and that protect attacks. All could be loaded on to wall defenses.
Barbarians: Dropping Rocks
Archers: shooting arrows down
Wizards: casting different spells (like lightening, freezing troops in place, or regular attack)
Hot tar dropped on attackers that lingers and causes damage.

Barbarians: switching out different equipment
Sword
Shield (plus defense)
Mace (plus attack)
Lace (plus range and attack)

Wizards:
Cast different spells for different kinds attacks
Lightening
Ice
Fire
ect.

Giants:
Wooden Clubs
Club with a nails in it
Long Sword

Wall Breakers:
Hooks and Rope
Ladders

Archers:
Crossbow (plus attack)
different kinds of arrows

Balloons: Drops heavy equipment like an airdrop
Catapult
Log for ramming walls
Shields

Dragon Riders:
Barbarian King
Archer Queen
Sorcerer

Just a few ideas to start. What do y'all think?