A Laboratory for Defensive Buildings
I've read over the list of ideas that have been mentioned and the following rejected list and didn't see this one.
Engineers Hub
It would be kinda nifty to have a building similar to the laboratory for defensive buildings or an engineers hub. It could also be combined with the siege weapon idea and have an increase to the defensive buildings vs the siege weapons.
Upgrading the buildings already increase basic effects of range and damage but having a building that increases resistance to lightning spells or the freeze spell.
A place where you can increase your defensive buildings damage and range for a duration with a gem cost.
Expanding upon other ideas, enable the arrows to be flaming or the cannon balls to slow. The wizard spells be lightning to combat the PEKKA knight or the X-Bow to have a slow effect.
It could also be used to incorporate upgrades to towers in the form if traps on the towers. Like the inferno tower can burst into flame and the cannon can explode.
Instead of having multiple places to have to re-arm or initiate multiple effects it would allow one place to initiate changes of a special nature.