Inspired by:
-Node 113105: Ultimate Costume Shop by Holy-Nightmare -- Fangel's comment #7.
-League of Legends Mystery Gifts.
So this idea is fairly simple. In the history of SK, there have been 66 prize boxes according to the wiki page. Of those 65, boxes (57 only obtainable by using real-word currency,) only 5 are currently available. There are also 8 Lockbox variants, of which only Mirrored and one other are available at any given time. This means that most cosmetic items are unavailable for purchase except from the limited number unboxed when they were available.
OOO should introduce a Mystery Prize Box, permanently available and purchasable for $5. Buying the Mystery Box would give you one of the previously available prize boxes. For example, I buy a Mystery Box and it gives me a Hunter Prize Box. That box's drop table has the items that were in the Hunter promotion. This would be a good way to slightly increase the amount of old costumes and accessories without tanking their prices, while counteracting the fact that those old costumes get rarer and rarer every day with the players who possess them becoming inactive or even deleting their accounts and items permanently.
I mean you've quoted me in the OP with pretty much the same words, but yeah. Having a sort of mysterious prize box would be wonderful, and heck, would be great because if it worked with all future promos too (meaning if you buy a box before a new promo when the new promo comes out there's a small chance the box you bought before the promo can be one from the new promo). Heck, you could even have mystery boxes include surprise boxes from previous years, meaning you open two boxes to get a box, since surprise boxes are yearly and have all promos up until that point, meaning surprise box 2014 won't have gem promos.
With the addition of mystery boxes, it opens up one more avenue to buy boxes in, and since there's no bundle for them (outside of maybe when they're introduced), it still encourages players to buy into promos they're interested in.
I would say having mystery boxes be purchaseable individually for $5 or bundled with $10 energy purchases, both, would be a good way to bring in a bit more income while keeping players not as angry at the same time.