You could do a pun-style thing instead of a literal costume? Like, be a bridezilla by dressing as a dinosaur but wearing a veil, or a bridesmaid by wearing an apron over a thrifted wedding dress and carrying around a feather duster, or go full 70s hippy as a “flower girl”? A grizzly bear blinged out in gold rings?
You could do a pun-style thing instead of a literal costume? Like, be a bridezilla by dressing as a dinosaur but wearing a veil, or a bridesmaid by wearing an apron over a thrifted wedding dress and carrying around a feather duster, or go full 70s hippy as a “flower girl”? A grizzly bear blinged out in gold rings?
Get a used white dress and stain it with red wine (the guest who wears white and gets a drink tossed at her).
Wear a bridesmaid dress (there's plenty at thrift stores) with a sign that says "always". Voila you're "Always the bridesmaid".
The one that pops to mind is Miss Havisham from Great Expectations. On a more normal route: flower girl.
Crying child Annoying aunt with the camera Anyone elderly Wedding crasher (can’t say i know how you’d dress for this)
How about an 80s bridesmaid? Raid a secondhand shop for the biggest most ridiculous pink fluffy dress.
Adam Sandler’s wedding singer costume? Elvis impersonator celebrant?
The drunk minister, an overgrown flower girl, redneck wedding outfits - camo formal ware, tragic bridesmaid, overdressed nouveau rich relative overly flashy...
Royal wedding guest - quite a formal outfit, pref tailored with a crazy headpiece.