So last night I played through some KHUx chapters and watched some videos on the story as I was a bit confused when it shifts to post war. (Shout out to Damo for some great vids on the subject.) 

anyway during some of the clips I noticed the MoM tell Ava that she is collecting hope in form of the dandelions. At first I thought this might just be a throw away line. But then playing through some chapters when Ava recruits you as a dandelion she tells you and Skuld that again the dandelions are hope. 

 

In KH3 we are told the black box contains “hope”

Perhaps this means that in some way the dandelions are contained in data/book form in the black box. 

Luxu is shocked by MoM stating what is in the box. “Well ya see it’s the hearts of all the keyblade wielders” (but the keybladers are still around duh! I dunno, but Nomura logic could make it work. Maybe that scene is in the KHux universe and set after the war or something)

This could also explain why he can’t open it. He needs to keep them safe in the box until the new world has been created. Essentially MX almost continued with MoMs plan of a new world and unbeknownst to him the dandelions become the new leaders of that world. 

I dunno this is just a though. Maybe it’s an intentional red herring. Has anyone else noticed this? SORRY IF EVERYONE HAS ALREADY DISCUSSED THIS AND IM JUST BEHIND!! Anyway what do you guys think? I know there’s lots of wholes and I’m not all in on the theory. But those were just some thoughts I had. 

Interesting theory! I've never thought about the idea that the box somehow "contains" the Dandelions, but I think it holds some merit.

After recently beating KH3 on critical, I was paying closer attentions to scenes than on my first playthrough now that I had some context. One thing I noted was the scene when Luxord mentioned the box contains "hope", like you said. A few scenes after that, Sora says something to the effect that "hearts are hope". This reminded me of another theory floating around that the box somehow contains MoM's heart, which is what's going to allow him to return. But containing the Dandelions "hearts" through some data-copy or something would also make sense regarding that connection.

I'm very excited for the future of the series, hopefully the months leading up to E3 will have KHUx updates with some more clarification on where the story is heading, along with the upcoming DLC.

To be honest, I was suspecting something similar. I mean, if the Dandelions and the others are inside a Datascape, they must be inside a computer; but where is that computer. Inside the Black Box?, might be. But how does the computer get enough energy to keep working inside that box, if that is the case?