Not giving every character from every arc a costume but a few from each instead.
DotC Shadowstar: just a black blanket and she calls it a Tall Shadow...get it?
Riverstar: a fish or a mermaid
Thunderstar: Lightning Tail *cries*
Windstar: nothing but says she's the wind before procceding to zoom off
Skystar: Gray Wing
Gray Wing: a mountain (he comes down from StarClan and sits on Skystar for a bit)
One Eye: a scary pirate
Star Flower: as the Blazing Star herb
Moth Flight: a big green or white moth
TPBFirestar: a shooting star or a firefighter
Graystripe: a waffle (Firestar's doesn't like waffles meme for the win, but he does still like Graystripe though)
Ravenpaw: a cowboy
Barley: a farmer or a cow (either way, he wants to kinda match with Ravenpaw, but not fully)
Bluestar: nothing but sings the
Blue song by Eiffel 65 on repeat so it makes sense
Whitestorm: a ghost (just a white blanket with holes but it somehow still looks good and is indeed effortless lol)
Tigerstar: a tiger
Lionheart: a lion
Redtail: a fox
Scourge: a panther or Batman
Silverstream: a pancake (because she wants to match with Graystripe and annoy Firestar)
Sandstorm: a sandstorm (somehow, no idea how exactly)
Princess: a princess (dress, tiara and everything else)
Brightheart: a badass pirate captain
Cloudtail: a cotton candy cloud
Cinderpelt: a firework (because she has a great personality)
Yellowfang: a grumpy granny
Spottedleaf: a perfume ad
TNPBrambleclaw: a bear
Squirrelflight: a squirrel
Leafpool: a leaf
Crowfeather: a very grumpy crow
Tawnypelt: a badger (because Midnight gave her the idea)
Feathertail: a feather duster
Stormfur: a storm cloud
Mothwing: a big golden moth
Hawkfrost: a mermaid (because we all know he would)
Leopardstar: a leopard
Blackstar: a whiteboard (because he's too white and nerdy according to what Mama Tad said)
Onestar: a Firestar lover that turns into a Firestar hater midway through the Halloween Gathering
PoTHollyleaf: the warrior code
Lionblaze: a lion
Jayfeather: the stick
Breezepelt: a black storm cloud
Nightcloud: a black cloud (basically, her and her son's costume would be practically the same)
Ashfur: an emo (with red streaks in his fur) or as the Moonkitti meme of him basically being Rapunzel
Sol: a lion (because Hollyleaf thought he was upon first seeing him)
Purdy: as nothing but he tells scarily long winding stories instead (so like usual)
Mistyfoot: Stonefur (she does so every year just to make Blackstar and Leopardstar uncomfortable)
Berrynose: glued a berry on his nose and constantly makes jokes that his costume is "too on the nose"
OotSDovewing: a dove (or a pigeon but saying she's a dove just to mess with others)
Ivypool: Hawkfrost (which offends Brambleclaw)
Cinderheart: Cinderpelt
Tigerheart: as Tigerstar as a joke (but then sees the uncomfortable/shocked expressions of others and feels bad)
Mapleshade: nothing because she already is scary
Brokenstar: a demon (not much effort since he technically already is one as a Dark Forest spirit)