Sinclair's Copy is already one of the highest-skill abilities. Similarly, an item that copies base abilities would force users of Mimicry to pick slots and targets based on the enemy team's composition.
Haze imbuing 3 can temporarily steal Vyper's slide passive (Slither).
Lash copies a vindicta...
I can see this working since for a 1250 item it would be too strong, even with the 50% suggestion since the passive is so strong. I would say making it a active with abit less stamina recovery would make it much more balanced. I think it's a great idea though!
Here's my suggested changes ...