Garlean Inability to use magic.

[KageTokage]

It's stated that the Resonant procedure was originally designed to simply enable the Garlean physiology to use magicks, so I'd assume there's a physical component to it on top of the soul-altering aspect that imbues the subject with an artificial Echo.

Assuming we get a proper Ilsabard/Garlean-focused expansion to tie up loose ends, I'd be really surprised if the subject didn't come up again as I'd think the prospect of creating more Resonant-enhanced soldiers would've been very appealing given how effective it was with Zenos. Its utilization in the Bozja storyline was...disappointing to say in the least, as it was basically just a throwaway plot device that didn't consider any of the deeper implications like the human resources involved.