Maybe have a building that upgrades wizards into summoners, who summon a boom that makes booms? (like the two stage fireworks?)
I'm thinking of adding ways to upgrade buildings which would unlock new functionalities. My idea was to make each Arcane Overload generate new currency, Orbs, which could be used to other stuff but changing units into new ones is a cool idea - I'll think about it. :)
Summoners could summon some creatures that shoot fireworks and disappear after some time and you could upgrade how many fireworks those creatures shoot.