Theory - I know what's inside the Master of Master's box. *SPOILERS*

[Luminary]

What if there is nothing in the box, but something will go into it when opened? I’m thinking like a reverse Pandora’s Box where it would either suck in all the light or darkness in the world. Alternatively, it could be a magical, perfected version of Ansem the Wise’s Kingdom Hearts Encoder that can contain Kingdom Hearts within it.

 

[Seighart]

Another way to look at it is when KH3 comes out, im expecting a scene where Maleficent is shown entering or digitizing herself into the BoP and says these same lines, but not until the later half of KH3. When we see it many will probably go "Oh that happened in Union X!".