I think it would be great if pure towers have their uniq abilities.
X% for critical strike (may be only on main target)
X% chance to poison (not stackable) for Y seconds
microbash on main target for 0.X seconds and Y damage
X% slow for Y seconds
X% chance for chain strike
-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
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.