Great idea!
As for the primary antagonist, I was going to have it as like his deepest fear realised in an imaginative fairytale way - like a dragon or something but I feel like this is a bit cliché and I was running out of words so I wanted to suggest that his primary obstacle is actually his own inability to imagine the world as described by the words of the book. Visually, I imagine this actually being represented on screen by him existing in the white spaces of a book where an illustration would go and it's only populated by things when he imagines they're there. The words on the page exist until the final confrontation, when everything is blank. I guess up until that point, the thing at the end is "the most terrifying monster ever imagined" as that leaves it up to the protagonist to imagine.
Still kinda working through this one, it's an interesting one!
Thanks for your comments, as always.