I have some ideas for additional units.
Priest: Increases the health regeneration of nearby units
Hero: A soldier that moves across the path, untying captured units.
Mage: deals ranged AOE damage, has a low attack rate.
White goblin: splits in two when killed
Goblin king: a black goblin wearing a crown, has twice the health of a black goblin and deals twice as much damage. Can carry multiple bound units.