The very first part of what would make the whole of Welcome to the Glass Castle was the power of the stranger in Andrea's body.
I came up with the basis of the power a few years ago, before I even heard of Worm, and later retooled it so that it would function within the world of Parahumans, and left it in a google doc to use if the need or opportunity arose, along with a handful of potential name - Dance Macabre was my favorite of those, and although it's not actually used in the story, it still is what I mentally refer to this character as, and what I will call her for the rest of this retrospective. Dance Macabre's power is as such: a person who does not possess a singular body but is a hivemind of maggots, breeding themself inside corpses and controlling human ones, with the precision and dexterity of the control increasing alongside the amount of maggots inside the corpse, able to regenerate the hivemind fully from one single worm. To balance the power, it was determined that the total mass of maggot would be limited to that of the mass of one human body, and the nature of the power implied a case-53. Indeed, that was the supposed backstory of Dance Macabre: a case-53 finding Andrea's body after she was killed in a hit-and-run, and attempting to take her life for herself. Unfortunately I couldn't find a way to fit Dance's Macabre origins and motives within Valerie's limited perspective, which I do strongly regret. That said, although Dance Macabre's power was technically what I came up with first, the true seed of Welcome to the Glass Castle, and the source of its themes and general aesthetics, was Valerie's glass powers.
I do not have a specific trigger event for Valerie, but I did base her power on a very specific mindset and emotional state. Valerie's power is one of loneliness, of isolation — not the loneliness of being physically alone, but the loneliness of being with other people while being cut off from them. An example that comes to mind would be that of a closeted queer woman at a family dinner, listening as everyone around her complains about or ridicules people like her, unaware that she is one of those people, both protected and alienated by her anonymity, her invisibility, the very thing setting her apart equally invisible. She can speak up/come out, which gives her the tools to openly defend herself, but also would be seen by other as an escalation, as causing drama and hurting those around her, and opens her to direct, targeted attacks. Valerie's power is this made physical: the glass represents a twofold protection, both in term of physical bareer and in that it has a stranger element making her invisible while behind intact panels, but at the same time, the glass physically isolates her from others, and to attack she has to break the glass, which removes the protection and presents the risk of hurting herself on the glass shards she uses to attack. This is where this story comes from: this power, and the passing idea of the Welcome to the Glass Castle title, which I liked enough to decide to write a story for it.
I decided to use the Dance Macabre power in conjunction with the glass castle power because I liked the idea of the contrasting aesthetics of glass and decay, the contrast of a clear pane of glass and the rotting, maggot-filled corpse hitting at it. Although Dance Macabre did end up shaping the plot of the story and its horror elements — a loved one dying and being replaced by its puppetted corpse, the slow realization that something is wrong —, the actual 'maggots' aspect power was a lot more present in the original storyline, which notably included Valerie making a first failed attempt on Dance Macabre's life, and Dance Macabre retaliating by essentially sending a dozen of zombies at her.
Welcome to the Glass Castle was, in my original plans, a lot more ambitious, but ended up... shrinking, for lack of a better term. I initially meant for this to have a nonlinear narrative, starting in medias res (with the aforementioned scene of zombies!Dance Macabre trying to break through Valerie's glass fortifications) and ending on what would chronologically be the first scene (Andrea's death, in her own point of view). However, I ended up doubting my ability to pull nonlinear narrative (from experience, the microchapter styles makes it easy for things to be missed by readers, even when explicitly and linearly stated), and went back to something chronological, which led to a streamlining of the plot (mostly, the zombies being dropped).
I don't think the "shrinking" of Welcome to the Glass Castle was a bad thing (except for the loss of on-screen motives for Dance Macabre). From experience, longer, plot-heavy stories are harder for me to pull-off, and the tighter focus on Valerie's internality allowed me to take space for imagery, which was a lot of fun. I loved the original idea, but I'm not at a level where I could have done it justice, and that's fine. I love what I did write. I hope you liked it to.
Either way, don't hesitate to tell me what you think.