Quite literally any form of armor piece can be used a costume. Is there any concrete reasoning for costumes being simply costumes and not fully functional armors and helmets? What star-rating, stats and maybe bonuses do you suggest they would give if they were actually combat-ready equipment?
I'm highly conscious of the fact that OOO will very likely not turn costumes into armor pieces due to the nature of the items and the effort of retuning each piece and individually giving them stats and balancing them overall, so I believe this will not fit to the Suggestion board. However, should this be more appropriate to the Arsenal section, feel free to move it. Alternatively, direct me to any similar thread topic if there are any: the search engine is unnervingly shaky.
Costume slots and costume armors and helms are for looking smexy n' stuff. Need I say more?