OP here with a minor update
We now have specific weight gain headcannons for each of the ladies from Three Houses!
These headcannons are not by me, and were crafted by one of the writers who’s working on the project, and are as follows
'Shameless Hog: Hilda, Hapi (Hilda’s inferiority complex towards her brother and willingness to abdicate all responsibility are prime grounds for her to become a rather shameless fatass. Not to mention how much she can try to milk sympathy 'oh I'm just so fat, please help me~'. Hapi’s supports with Constance show that she's a glutton, and her persecution complex towards the Church could be construed as a 'ha ha, I'm eating all your food, suckers,' revenge.)
Prideful/Bashful about their weight: Dorothea, Constance (Night) (Rather straightforward cases for the both of them. Dorothea would of course outwardly project that she's proud and confident about her growth, but her deep insecurities make this a thin veneer. Similarly, Constance would stubbornly spin her weight gain as 'a brilliant plan, mark of nobility, etc.' until something actually embarrassing happens involving her weight.)
Smug about her gains: Cornelia, Kronya (I am assuming out of my own bias that we are taking 'true' Cornelia and not her Argathan impostor. Either way, Cornelia dresses exactly like a woman who knows she is sexy and isn't afraid to flaunt it. If every woman of the cast is getting fat, I see no reason she wouldn't keep that attitude. Kronya is rather more straightforward, she is an Argathan and inherently believes she is superior to everyone around her.)
Confident of her size: Anna (Anna is first and foremost a merchant, not a combatant, so there is no reason whatsoever for her to feel ashamed or inconvenienced by her weight. In fact it gives her all too many reasons to 'flex' her size on the others.)
Content: Mercedes (Mercie is the uncontested 'chill' girl of the cast. She dislikes fighting and physical exertion to begin with and prefers baking/cooking in all respects.)
Oblivious to her gains: Flayn, Catherine (Straightforward again: Flayn is rather endearingly oblivious to a lot of things, despite her surprising wisdom on many topics. Catherine is more a case of 'she's hit her head too many times' to have the awareness to recognize, in my opinion.)
Self-Deprecating for 'letting go.': Manuela, Ladislava, Leonie, Rhea, Constance (Day) (Manuela built her entire songstress career on her beauty and her voice, slipping on the former would only exacerbate her self-destructive tendencies. Ladislava and Leonie are 'soldiers,' the former putting her ability to fight for Edelgard above practically all else while Leonie desperately wants to emulate Jeralt. Of course they would abuse themselves over 'failing' to live up to their own standards. Rhea would certainly be a more subdued case of it, but given how she already feels that she is unworthy to guide Fodland in Sothis's stead getting fat on top of that would make her miserable. Depressed Constance is already self-abusive, getting fat is just another whip to beat herself with.)
Embarrassed by WG: Edelgard, Bernadetta, Annette, Petra (Bernadetta is straightforward, even if her shut-in life would logically lead to her getting fat anyways she's still going to freak out over it at the slightest mention given her trauma. Annette is going to struggle with it because getting fat dulls her 'giving it 110%' attitude, makes her look lazy, etc. Petra comes from a backwater 'warrior tribe' nation, gaining weight makes her suck at hunting and fighting. Edelgard, as always, is a debate topic, but I think it most reasonable to put her here because getting fat would 'make her look soft/weak' and Edelgard cares about her image quite a lot.)
In Denial about WG: Shamir, Ingrid (Shamir is a mercenary first and foremost, getting fat would be an insult to her 'credentials' so she is going to deny it as long as she can, and as an archer by trade she has more 'room' to be insulated from the consequences. Ingrid on the other hand defines herself around her desire to be a knight, and as being a fatass runs directly counter to that dream she will deny it as much as she can.)
Depressed by WG: Marianne, Judith (Marianne needs no real explanation, she's already depressed about her own existence. Judith is rather more interesting: she's going blind, which already means her fighting days are coming to an end. Getting fat on top of that is as much of a hard stop to glory as you can get.)
Annoyed/Enraged by WG: Lysithea, Sothis (Self-explanatory. Lysithea is the type to get super annoyed over the consequences of her sweets-gorging despite refusing to stop, and Sothis has every reason to go "HOW am I getting fat? I don't even have my own body!")
"Obsessed" with WG: Monica ('Obsessed' in at least one respect, she needs to be as fat as Edelgard, given her entire personality revolves around Edel. Shez similarly falls into this category, but mostly out of her rivalry towards Byleth [she HAS to be bigger than her in order to be better than her].)
Unique Case: Arval (Arval would both tease Shez for putting on weight, but also egg her on, since in game they often add to the fire that is her rivalry with Byleth. As for how they feel in regards to their own gain by proxy of Shez, they’d probably not be entirely against it, and might even take some enjoyment out of it to further help Shez accept her weight gain as a method of being better than Byleth.)
Player/Reader Agency: Byleth (At the end of the first in-story month, after Byleth has put on her first noticeable amount of weight, the player/reader will be given options of how she feels about her weight gain. The choices will be something along the lines of “I like it (positive option)”, “This doesn’t really bother me (neutral/more apathetic option)”, or “I don’t like it (negative option)”.
As far as progress goes, plotting for the Black Eagles and Church routes should hopefully be starting soon!
We are still looking for writers to work on the Blue Lions, Golden Deer, and neutral routes, so if this project interests you, then let me know!