I did this kind of spur of the moment to get it out of my head. While I'm NOT open to commissions inspired by this (Not right now that is), I figure I'd post this in case any of you want to use this list of ideas for fun.
Before you go "Aren't some of these redundant?", it's semi-intentional, as this list is made so that you emphasize that particular element in that image and go wild with it. Besides, many of us have more than one item of clothing that we use for said occasions, so why not?
For the really vague ones? Just draw what YOU and your character of choice associate with the word and draw that, no need to worry about anyone policing what is or isn't correct, The vagueness is also to help make this list as flexible for characters of all shapes, sizes, genders and cultures to have room to be creative there.
Why "costume" instead of "Cosplay"? Because not all characters exist in universes that are familiar with real world pop culture. Not to say you CAN'T use that to draw cosplay, mind, just means you don't have to focus exclusively on that.
Do you have to draw up all of them? That is completely up to you. If I were to open up to commissions using this theme myself I would, indeed, do all of them, but don't worry about having to stick to that rule yourself.
So anyway, here's the list, enjoy:
Dress clothes/semi Formal
Dots and circles theme
Letters and text theme
Stars and space theme
Fictional brand name
Out Of Character