I think it would be great if pure towers have their uniq abilities.
Pure Fire:
X% for critical strike (may be only on main target)
Pure Nature:
X% chance to poison (not stackable) for Y seconds
Pure Earth:
microbash on main target for 0.X seconds and Y damage
Pure Water:
X% slow for Y seconds
Pure Light:
X% chance for chain strike
Pure Darkness:
-X armor (not stackable) for Y seconds
Some of these abilities can be already used in double or triple towers, but maybe replace them with the new ones? For example
Glaive tower:
Each attack bounces between creeps for maximum X times, damage reduction per bouce Y%.
Creep Type: Destroyer
When creep dies, all towers in small radius can't attack for X seconds.