A gem or item in the cash shop maybe 1500 Crowns or something when you use it it opens up a sort of Dye and equip menu from there you can equip any piece of gear you have in your inventory (no matter the tier weight or otherwise) in any combination then dye it as you please once your done hit finalize the costume gem item is consumed and you get a costume copy of what you made added to the collections page.
this way certain looks still have to be worked towards if its not craft-able. not to mention i can see this adding a level of customization that would be refreshing.

can't begin to tell ya how tired i am of making an awesome set then having to replace parts with really ugly pieces and ruining my overall style i wanted. would also save ZOS from having to make random new costume combos every odd month when players can just fork it over for a higher price to make a personalized costume. i mean sure they can still do poly-morphs and things like wedding dress and other non dropped or crafted costumes. mount skins, pets etc.