Harley Quinn (DC)
The Joker
also another super simple cos idea. can buy a suit or improvise with any Joker-suit-themed shirt or outfit, wig can be used OR colourful hairspray OR an editing app such as Meitu or FaceTune2. add some makeup/facepaint onto that, and that's basically it. the cos can stand alone like that or some other props/features can be added onto that. yellow teeth not required, lol.
L (Death Note)
this one is also really simple. a costume can be worn or just wear a white t-shirt or dress shirt, some dark pants, and either a wig, dark colour spray for hair, OR edit with an app, and contacts can be used for the eyes, or they can also be edited in the app. add some dark eyeshadow or smudge some dark eyeliner around the eyes. easy peasy.
Homura Akemi (Madoka Magica)
also kind of simple. she has long black hair, which is either in braids or straight down - a wig or extensions and clip-in bangs can be used, black hair spray can be used or an editing app for the hair colour, purple contacts / circle lenses can be used or edit the eyes as well. a white dress shirt (with or without a beige/taupe/white jacket or sweater), a black or grey or dark plaid skirt, dark leggings or tights, and a red bow can all be worn to simulate her casual outfit, or a costume can be purchased online.
I plan to add more to this list soon!