So my main theory is that Henry is going to become the next Author by the end of the season/show. Face it, he is the one who has cared about the book since the beginning of the show and been deeply invested in seeing happy endings given out.
He is especially qualified since he is the only one who doesn’t see distinctions between villains and heroes. His mother is the Evil Queen and the Savior. To him, everyone is a shade of grey. He’s the perfect, impartial author the book needs.
I think the ending to the whole show is going to be happy, maybe not entirely so (I expect more loss before this whole thing ends) but I see the camera slowly drifting off to a quiet corner where Henry sits with the book, quill in hand, ready to begin his job and start inking the first lines in a perfectly blank book. After a while, he has just the right words to say and we hear him as he scribbles it down: