I'm just trying to wonder what such an item actually needs. It's stats don't appear to be problematic for its cost. Is it just that people don't find it's active to be useful?
The only inherent problem seems to be that it's accessibility goes down with the more turrets that go down. Though I could imagine a way for compensating for that, by making it's stats scale with downed turrets. (Not sure if that's a workable solution though)
As for you suggestion, I did have the idea that it's effect could potentially be a dual cast. Buff and Ally Turret or Nerf an Enemy Turret. (Inspired by the Garrison summoner spell from the old Dominion mode) Though I would imagine that being hard to balance.