(oh my god i haven't been here in forever sorryyyy)
YESSSSS THEY WOULD
Norrie would dress up as whatever is left in the stores OR somehow if she had the time would make her own costumes LOLLL

The idols are still quite new to Halloween! But luckily since their debut they all had a chance to have whatever ideas they had for a group costume! Last Halloween the girls dressed up as the Battle Chatelaines (despite then being a group of four but it works)

Omg I love this question because I've shitposted about Rocket having a mandatory rule for everyone to dress up during Halloween EVEN if she dislikes to.

She usually wears anything fluffy; Mareep, Whimsicott, Wooloo, and so on so I'm trying to think which one she'll choose this year,,, >:)

Mary lives and breathes Halloween. She’s had tons of costumes over the years, one that stands out is her harlequin costume with Aegislash elements.
Jin ofc gets pulled into the Halloween spirit with Mary, tho she usually just lets Mary come up with her costume.

Wily would have to be talked into it, but he's probably overdo it, like some needlessly complex costume using holograms or something. (Maybe he's dress as a suspiciously accurate The Shroud, since he helped make his suit).
Polly would probably go as a witch or vampire, to push the goth look.
Sivrit would dress in full Draconid armor.

Given the less than favorable view some of his local religious officials threw on him when he was young, whenever he has the chance he likes to dress up as "things that would upset a church" (so-called demonic entities, historical heretics, stuff like that)

And if he can find the friends for it, he likes to do some milder themed group costumes referencing some form of media, such as-

https://twitter.com/FSlip73/status/1470784453664514052?t=hryCGienBf7Rl4PGnfocXg&s=19

YESSS BRIAR LOVES HALLOWEEN, she always dresses up as something different each year, and she likes the sexy bunny suits for Halloween a whole lot DHDKDHSKDHD And also the typical stuff like bloody bride or mad nurse or even zombie and that sort of stuff !

She also has a Chandelure-inspired Halloween costume that mixes elements of a masquerade theme with some Phantom of the Opera because she likes to be DRAMATIC

But yeah Halloween is her favorite time of the year. Not only she can dress up as stuff but she can also scare children and the Macro Cosmos employees without being called a psychopath /hj

Avian canonically has a hydreigon sygna suit. He's not big on the season but if others are dressing up then he would as well. He'd try and lean more into the spiritual/horror aspect but often he'll try and match the vibe. Arryn has a large wardrobe. Everything from legally distinct pop culture outfits, monster outfits or pokemon cosplay like lopunny. The latter she doesn't wear so much nowerdays.

Stahl doesn't like halloween, thinks it's dumb and would rather have a meal out rather than dress up.

While Elio's not really big on Halloween, he would cosplay for the sake of his friends who are into the holiday - especially if they coordinate a certain theme beforehand.

Off the top of my head, he would definitely dress up as either: Al-Haitham (Genshin Impact), Belial (Granblue Fantasy), Yu Narukami (Persona 4), Zero (GARO), Adam Taurus (RWBY), Taion (Xenoblade 3), or Dan Heng (Honkai Star Rail). Their character designs catch his appeal. It's subjective to change, however, depending on the agreed-upon theme.

He dressss up as a lot of things, all depends on how he feels. Though his favorite costume has to be a Mega Ampharos, which his Dragonite dressing up as a Charizard