Note: I plan on publishing this as it's written through fictionpress.com or some other platform, so if you want to be surprised, this is your cue to leave.
I'm planning to follow Dan Harmon's Story Circle and do a pretty linear narrative with relatively few viewpoints and two main characters. So, with that in mind, here's a plot breakdown.
1. A character is in a zone of comfort.
The first protagonist is Henry, a boy who's sixteen for the majority of the novel. The opening chapters detail him being raised by two dark wizards and a friendly witch, and serves to get some of the exposition out of the way. The second protagonist is Sofia, a princess who's sixteen for the majority of the novel. The opening chapters detail her being bossed around by her little brothers, walked around with the house spirit, and eventually getting kidnapped at the age of five (by Henry's parents).
2. Something ain't quite right
Henry is a gifted dark wizard with a whole bunch of prophecies swirling around him. He wants to know his real parents, and while he loves his fathers, wants to go off and have an adventure. Sofia doesn't particularly like being a princess, especially with the restrictions placed on her, and so has decided to see whether the Foresworn Sisters would be a good place for her to go.
3. Crossing the threshold
Henry gets the call to adventure in some way; another prophecy manifests, or an orphan runs away to the farm where they live, or just decides to run away. He goes to the orphanage that he was taken from, looks up his file, and then runs away when the Sister cottons on to the fact that he was the orphan that disappeared so many years ago. As he runs away, he bumps into Sofia, and follows her on an adventure (in part because he's smitten with her). (This threshold needs work - it should ideally be a transformative moment for both of them. Maybe for Sofia it's a dalliance with magic? Not wholly decided on whether Henry recognizes her from his childhood, but she for sure doesn't recognize him.)
4. The Road of Trials
They pair up and travel north, to where they both have business. Henry is searching out his birth parents, while Sofia is searching out magic (though she doesn't quite know it). There are a number of different types of magic within the world, and we get a taste of all of them. Naturally, the Sisters are chasing down Henry (since he's the child of prophecy) while the royal guard is chasing down Sofia (who is a princess on the lam). Eventually both of these subplots are resolved, and the pair make a clean getaway, having other adventures along the way and picking up new skills.
5. Meeting with the Goddess
Henry and Sofia have their idyllic time together. They fall in love all along the Road of Trials, but here they both feel it keenly - a throbbing, teenaged love that can scarcely be contained by their hearts. An unspecified amount of time has passed, perhaps a year. Just as things are going merrily along, the magical crown of Donkerk plops down on Sofia's head, marking her as the new Queen of Donkerk. Given that she has two older brothers and her father was alive and well when she last saw him, this is cause for alarm, and the pair decide to sneak back to the capital in order to find out what has happened.
6. Meet Your Maker
The villain behind the happenings has been revealed; it's Sofia's brother, an illegitimate prince of Donkerk. He arranged for his father and brother to die, thinking that the crown would pass to him (and not knowing that he was really a bastard). This is the reverse Road of Trails, and things happen more quickly as Sofia and Henry use their newfound powers in order to infiltrate the capital. Tensions are thick between them, especially since Sofia's brother seems to have transformed himself into (or revealed as) a dark wizard. They both get beaten, hard, and barely escape with their lives. Maybe one of them gets captured.
7. Bringing It Home
Sofia and Henry (or possibly only one of them if there's been a capture) return to the farm where Henry's parents live in order to get help from them. Sofia attunes herself to her magical side. Henry finds a new reading to the prophecies that have dominated his life, or possibly finally learns the truth about his birth parents - some mundane truth rather than something world-shaking. They gather up what they need, and make a second assault on the capital.
8. Master of Both Worlds
This is the final showdown. The full wealth of magics are interacting with each other here, and new and powerful abilities are put on display. Dark (but not evil) wizards fight alongside paladins that move with lightning speed. There's a final showdown, with a monologue from the heroes, and in the end, when the villain has been defeated, they get married and Henry becomes the King of Donkerk. There are (possibly literal) fireworks.
In sum, Sofia's character arc is that she's afraid of being different and feels trapped, but eventually finds herself and uses that newfound power and freedom to win the day. Henry's character arc is that he feels like he has to hide who he really is, and feels a lack of love in his life, but eventually finds that love (and later acceptance) in Sofia, and through her, the rest of the kingdom.
And that's Part 1. More on this later as I continue to organize my thoughts.