Just having the whole plotline about Ead's magic and her crush on the queen that she doesn't seem aware of at the start and how she's torn between her duty to the queen and duty to the Priory would've been great! That would've probably ended up being one normal-sized fantasy book, and if she really wanted the other worldbuilding stuff she could've used the strategy of beginning each chapter with some in-world text (letters or textbooks or whatever) that would let us know about things, but Ead seemed to know a majority of what was necessary for the "Oh no! Evil dragons are rising and must be stopped!" plot anyway as best I could tell at like 350 pages in.