Great idea for a thread!

Jill: I have pretty strong feelings about her arc. She goes from someone who is very focused on herself and "survival" to someone who is willing to risk her own life to finally follow Aslan's signs. Jill's journey is a journey out of self and into selflessness. She also starts out the story as completely alone, crying and hiding, and at the end is facing down her tormentors flanked by friends and with Aslan at her back.

Eustace: I've honestly never really thought about Eustace having an arc in SC before. The thing that comes most to mind is when he and Jill shake hands and say goodbye when they are about to head out into crumbling Underland, not knowing if they will live or die. C.S. Lewis says it was the first time they had used their Christian names in addressing one another. So the development of his friendship with Jill—of growing more mature, burying hatchets and stopping bickering so much—is probably Eustace's arc in SC, imo. I wouldn't be shocked, though, if the filmmakers look at Eustace developing true friendship and rapport with Jill, someone he previously bullied, as a kind of atonement for his behavior during past terms at school.

Puddleglum: I don't really think that Puddleglum has an arc to speak of. What changes is Jill and Eustace's perception of him, not him as a person. They think he's such a wet blanket at the beginning, and he's really the best and bravest of them all. (Gosh, I love this character so much!) However, I think you've made a great point about Puddleglum saying from the outset of the mission that this'll be the thing to steady him and teach him to take a serious look on life. I can easily see the filmmakers developing this and making him more of a "quirky outcast" that eventually returns as a hero and finds acceptance. (I know there are scads of examples of this in film, but I keep thinking of Flick from A Bug's Life.) I hope they don't go that route because it's a pretty tired trope. (And the humor in it is that Puddleglum is not full of bounce and bobance and high spirits.)

Puddleglum: I don't really think that Puddleglum has an arc to speak of.

Yeah, based on the "finding yourself" comment from JJ, I think Puddleglum's arc will be more fleshed out than it is in the book. Hopefully not to the detriment of the character.
Since there was also mention of standing up to bullies, I wonder if Puddleglum will follow a similar line as Jill. After all, Puddleglum is the one to stand up to the LotGK in the book.