If you mean editing main characters to look like they are wearing costumes, you can use that "Cloud as Dancer" sketch from the general forum as one of the first ideas.
-Ramza as that Home Alone kid. AAAHHHH!
-Agrias as "Hentai tentacle victim." Draw a blob-like goo on her back, and some tents wraping around a leg, waist and shoulders, and arms.
-Orlandu as Obi Wan-Kanobi.... ...wait...
Meliadoul as the Green Goblin, for simplicity.
If you mean making actual holloween characters, then I would like to suggest a "Pumkinhead" monster.