Persistant Upgrades!!!

Range upgrades, attack speed upgrades, pierce upgrades! They're all so stupid and unoriginal! All these conception towers need them though! Otherwise, the towers wouldn't be balanced! How do we fix such a thing?

PERSISTANT UPGRADES!
Every X rounds, which changes for every tower and difficulty, you get to obtain a persistant upgrade. It all changes for every tower, but they're usually the smaller kinds of upgrades, like range or attack speed. If I wanted it to be like anything, they would have a cap limit just for balance.

What conceptions contain this idea?

 * tumbleweed rolls*