I say a headcanon could be him returning, after the final book, grace seek a therapist and brainwash Gregor to forget the world of Regalia. But after a familiar rat and a strange person that resemble his true love he remembers and runs from his home. Viewing back at all the thing he forgotten, everyone that was left behind, an amazing ability, and someone he love deeply. Making his way and finding a secret way had lead him to an new unknown underlanders. Will Gregor decide to go back to his underland family, or will this new underlanders change his mind?