Ah, I see. The post with all the pies confused me; I assumed the turrets could be made to upgrade on their own right, as opposed to into existing ones. My mistake.
Alright, no worries, I can think of more.
The Bola Launcher. This turret fires bolas (basically, two canonballs attached to each other with a chain) directly at other turrets. As all bolas are fired, they are fired at the cannon itself, wrapping around the barrel to distort it, causing the enemy turret to fire in random directions, resulting in possible misfire at its own allies. The bolas also cause decent damage to its target upon impact, and if the foe's condition is not resolved, the enemy turret may destroy itself. Bolas can be fired from the BL at any time, but the bolas themselves cost money. I'm not sure how that type of thing is handled in your game, but it is also possible to purchase explosive bolas that detonate a short time after attaching to the target. Use that idea how you wish.
Medusa Cannon. This turret can charge a special type of energy. It takes a while to charge, but the blast will essentially petrify it's target-the target is now nothing but a stone obstacle on the battlefield that cannot be replaced until it is destroyed. The shot is slow and has a slower rate of fire, but if it misses, say, a moving unit, the blast will create a large mound of rock where it hits, causing an obstacle for all turrets to bypass and takes a strong, concussive blast to fully destroy (mid-power attacks can break pieces off of the stone, but it takes heavy fire to get rid of it in 1-2 shots. The flame turret and related turrets have no effect.) Possibly another attack it will use is to fire its pertification blasts into the air above a stationary target. The mass will accumulate above said target into a large stone, which will drop; HARD. Medium and smaller-sized targets will instanly be destroyed and the stone will leave a large crater after impact, making a tough terrain feature for mobile units. As for large units, they too will be destroyed, but rubble will litter the impact site as opposed to a crater. This weapon . . . is slow. And expensive. But annoyingly powerful. Probably in the top tier of the turret hiearchy.
Groundhog Turret. As the name implies, it has the ability to burrow undergound, popping up after a period of time to pester foes with short range, machine-gun blasts. It will be vulnerable after each barrage, requiring time to cool before burrowing again. Each time it burrows it leaves a hole for mobile units to trip over.
Mole Turret. This is an upgrade for the groundhog (like an Adv. Groundhog Turret). In addition to heavier firepower from it's machine guns, it can also fire twin missles. Again, it has to recharge and cool after each assualt. It leaves a large mound of dirt when it burrows, a terrain obstacle for mobile units (hence why it is Mole Turret as opposed to Adv. Groundhog Turret).
Do these work better?