What? Fangel? Why do you want this?!
See I'm pulling a buzzfeed and making a clickbait title, but this isn't just to shove more exclusive items inside prize boxes, hear me out.
Instead of putting confetti in specific prize boxes (or in addition to), add a special material that can be used to craft an item from that box. Not costumes, as those are cosmetic, but say, you get a celestial shard from the equinox promo box. You use this shard to craft a 3* celestial blade from a 2* calibur. Each level of the item requires one of these materials, so if you spend $20 on an equinox promo you'll get 5 celestial shards.
Box materials would always be in each box so you'll always be getting at least one of them, and makes boxes with unfavorable odds more fun to buy (if they have special gear in them) as you will for sure be making progress with them, or you can sell this material to other players who just want that shiny sword.
Is it pay to win? In a way yes, but it's as pay-to-win as the current energy market is... That is you can buy energy directly from players who have paid for energy.
Reskinned items can still be included in the boxes as is, however new weapons with new mechanics/stats can be crafted via this method... And in fact, reskinned items could as well if RNG just really hates you. Want that squirt gun? Here, go get it - have a water balloon fragment. Want a mixmaster? Have a battle-worn batter beater.
Overall I think a system like this one would make prize boxes a more desirable item as they bring us new weapons that we as a community work towards (whales buy the boxes, sell the materials, we get fun weapons!... Or the whales can sell the fun weapons, either way it's a win!) instead of a limited number of rich individuals get to have all the fun. I don't want to pay 20k CE for a mixmaster... I want to craft one myself. I can do that by buying $20 worth of prize boxes and getting some fun costumes I can also sell to make back some of my money I spent. Now I have a mixmaster and some more in-game money. I'm happy. I didn't spend $20 to get what I didn't want anyways.
I'd be happy if it was materials to make cool reskins, SK gets a good bit of flak when they release box exclusive items. Yes it would make them more desirable and would be especially nice if this material was in every box. Then SK could make it so that players have to buy a certain number of boxes before they can craft these items.