Let’s go with some theories floating around-
Tomorrow is the last Soeul show, does he open the box and reveal all the members at this final SK show. Not sure if I buy this one.
Its’s tied to Jack in the Box and he releases Jack, his true self after his years with BTS, his post military self and inner exploration. Ok I kind of like this.
It’s a big nothing and he just brings it out as part of his stage presentation. Probably but we really need some insane delulu mystery behind it.
My favorite theory is that towards the end of his tour, his encore show in Soeul is announced, in June of course. He brings the box out as usual and opens it to release the members to reunite BTS.