Do you have something specific in mind you want to be? I'm also plus sized and LOVE Halloween (and theater...and costumes...)
Some ideas based on easy-to-find items:
red bridesmaid/prom dress=Queen of Hearts
Pink fluffy 1980s dress=Glinda the Good Witch or one of the good fairies from Sleeping Beauty
Thrift store wedding dress=Bride from haunted mansion (or zombie bride, or some other spooky bride...)
Pirates are easy to do and can be made with thrift store items.
Wicked witches are another that's easy. Black dress, hat, some funky accessories...paint yourself green if you want!
Or, what about Bette Middler's character in Hocus Pocus?
Disneybounding looks fun, too. I saw someone DB as Snow White. Blue shirt with red trim, yellow skirt or shorts or pants, red ribbon/headband. Pretty and easy.