Originally, Feedback was a soft-counter to Medivac drops if the Terran player was neglectful in artificially reducing its energy prior to attacking. Even if the Medivac wasn’t destroyed, it was often damaged enough that it could then be shot down with Stalkers, and similarly could be used to remove Vipers from the fight, which otherwise remain too far behind the front lines to really get hurt by any follow-up attack (as long as you don’t rush them forward with “Select all army”).
Rather than suggest just reverting the damage Feedback used to do (it used to do damage equal to the energy removed - now it only deals half damage), I would like to suggest a late-game upgrade that restores the damage and hence is a bigger tech commitment if you intend to use it to counter multi-pronged drops or heavy Viper or Infestor play.
Twilight Focus - Researched at Templar Archives, costs 150m 150g, takes ~71 seconds (100 units) to complete. Damage caused by Feedback increased by 100% (so 1 energy = 1 HP, up from 0.5 HP)
If possible, I would suggest giving the Feedback visual effect a red glow to indicate to opponents that it’s upgraded.
(The name is based on the fact that Feedback was originally a Dark Archon spell, implying that High Templars are learning to tap into the powers of the Void - thus with Twilight Focus, they seek to learn both disciplines in tandem, as Tassadar did)
Being on the Templar Archives, it competes with Psionic Storm, so is not something you can get quickly, and you definitely want to get storm first in most situations - the only exception is possibly when you’re dealing with a lot of enemy Phoenixes.
Thoughts?
(Possible additional restriction, if its timing needs to be kept in check or otherwise provide a bigger tell for the opponent… requires Dark Shrine, due to the High/Dark duality of Feedback)