I have a feeling that casting Song Joong-Ki as The One is perhaps to allude that Tree With Deep Root's King Sejong is a descendant of Eun-Som. I'm expecting Han Suk-Gyu to cameo as older Eun-Som.
I feel it too, like an RPG. And really, any of you who have been dabbling in RPGs will know that even GOT "borrows" a lot from (older) RPG lexicon. I guess this is why I am not bothered at all by the "unconventional" way the story play out: some are summarised and abridged, some are told in detail. I also think that there's an intended confusion as we try to learn and navigate through that world's folktales, fairytales, legends, and recent events, just as the characters do.
I do feel the "uneveness" of expositions, which are typical and sadly becoming a plague in saeguk, but here, I found them more tolerable (and hurray for inner monologues! ). So far, I am liking this for the very reason that this doesn't feel like the usual korean dramas. The director, especially, and writers aren't known for that typical k-drama/saeguk.