The series has so far been consistent in presenting a merging of souls as something that can't be undone. The part of Gideon that Harrow took, I don't know if she can give it back. I don't know if Gideon would even want her to.

Maybe I'm a romantic, but the ideal ending to me, the most satisfying conclusion, would be Harrow balancing the scale by giving a piece of her own living soul to become a part of Gideon. Not Gideon's heart returned, but Harrow's heart in return. We've seen lyctorhood as a one-sided consumption, and we've seen it as a mutual consummation. We've still never seen it tried as a reciprocal exchange.