A simple extra tab in character screen. Call it Costumes.
Make costume choices so you can save each costume. Buy extra costume slots.
And then all character gear slots with show/hide option so if u wanna play witthout show any chest piece u can do so. Put in the costume u want the look from. This is a system many mmo games use and it works great.
Why try make something "unique" when you have systems that works perfectly -.-