For RNG boxes, they should put in all out-of-season/style outfits, and make an RNG Box based all of those. If NCSoft for whatever reason discontinued outfits from the store, make them obtainable via RNG Box. No trash items, not even if its a brilliant key, JUST outfits.
So the players have a 100% chance of winning something good, but still have a motivator for purchasing them (like completing their Outfit Collection.) and will never feel like they're cheated, because they get what they want exactly, even if its not an outfit they're aiming for.