Prologue, Part 1 - Awakening
Artemis wakes Ryza up and the playable portion covers everything from that point to either the point where they meet Lancel and Axton, or when they actually leave the shrine. This would probably be an Escape map.
Prologue, Part 2 - Raiding Party
The playable portion here would cover the raid on the bandit camp to get Kelton's sister out. This would act as a much more stereotypical early-game map, having either Rout Enemy or Defeat Commander as its objective. Of course, most of the actual fighting would be done by green units, but still.
Prologue, Part 3 - Flight!
It's honestly difficult to qualify the escape from Castle Legerius as battle map material, considering it would just be an Escape map with barely any combat. Maybe it could function like a much more hardline version of Path of Radiance's prison break map, where Ryza would have to avoid getting spotted by night watchmen while making her way to Artemis's tower cell? I dunno, but then again, stuffing all this along with everything else that happens between Miau and the attack on the Talon camp into cutscenes would make for an exhaustively long sequence of non-interactivity.
Prologue, Endgame - Talonsbane
This map's objective would likely be "Sypha Arrives," with a turn count limit and several bonus objectives. These would be divided sort-of evenly between granting extra turns upon completion, and rewarding the player with extra items, weaponry, and other goodies (cough-cough Bernard). Worth noting is that this chapter's boss would start moving at some point, which would serve as a great scare for newbies and a bonus EXP opportunity for veterans. She could perhaps be carrying a droppable item for extra incentive.
Over half of the threadmarks cover story beats that a standard FE game would relegate to cutscenes or optional content in the case of the support convos we've had.
And of all that story, we've participated in exactly one 'standard' early-game chapter, that being the Miau bandit raid. Honestly, with all the escape chapters we've had to complete so far, we could be mistaken for a Thracia spin-off.
…thinking on it, the Tellius games would be the ideal bases for a romhack based off this quest, as a lot of the mechanics used in their Base Camp feature seem to have spiritual successors here. The actions we take to improve our skills is like assigning ourselves Bonus EXP, and our interactions with our allies would be covered by Supports and Base Conversations.