Ideas in the comments

Unknown: one of the Helltaker girls (idk who yet but I think it would be cute if her and red were Cerberus even though Cerberus is 3 people instead of 2)
Gimari: probrably natsuki
Marco: six from little nightmares
(Also I'm only putting the ones that would dress up or the ones that would be forced to dress up)
Sakura: Mae from night in the woods
Jay: Thomas sander's Deciet/Janus (so jay would be a lying snek boi)
Morality: some version/something like little red riding hood cause morality still likes children's stories
Bad creativity/bad: genderbend judgement from helltaker probrably
Insanity: some creepypasta but really realistic so it's actually terrifying
Kira: a vampire, like the pale makeup,the cape and everything (she would feel cool)
Kathrine: sayu from no straight roads or just anything pastel and/or sweets themed
Charlie: would probably be either quest!bendy or quest!felix cause idk (also maybe the genderbend versions of them but I think that charle wouldn't mind being a boy character for Halloween)
Rei (not me, my MHA oc): kokichi from Danganronpa or any "glitch" or .exe characters

Y e s N a t s u k i

Rain: either wouldn't dress up or would be forced to go so would go as chihiro from danganronpa or a "anti-Cupid" like character cause I think it's funny
Anxiety: would make cyan do those cute couples costumes with him
Cyan: would not want to go but anxiety would make him
Lily: would just wear a mask and have a fake weapon, so I guess a purge like costume???
Shyanne: Yandere chan, also will try to make jonny go as "senpai"
Jonny: wouldnt go even thought shyanne tries to force him