It was rumored and never confirmed, so... That's probably it. Unless they plan on announcing it later, the Disney Plus series rumor was just that -- a rumor.
If KH4 is going to be the conclusion to this saga, then KH4 at the absolute latest. The way it's been touched on now and again, that's probably going to continue until the last minute.
There's opportunities for it to be revealed in UX's last chapters and Dark Road, but that would have to involve Luxu being around somewhere. So Dark Road is also likely. But knowing how this series is, it's probably gonna take KH4 to actually say what's in it.
The Master of Masters didn't want it to be opened though, so... Whatever's inside would have to be something that's problematic to the Master in some way. Luxord said that apparently Xigbar said the box contains "hope" though, so...
And not gonna lie, I think most people want another Days game for the sake of it. There's nothing wrong with wanting one, but this is how the series gets "convoluted" with "too many games" if there's no actual need for them. Same with BBS Vol. 2, if there's no actual journey of value to be had there, then it's gonna do more harm than good. And people already have issues with Melody of Memories being a rhythm game with lore in it.
Cause I know how this is gonna go, they'll make a Days 2/BBS Vol. 2, it's not what people wanted, and then they're gonna say "Oh why wasn't this just a cutscene or two in the next game, we didn't need this. Fucking Nomura and his cash grabs."
I agree with most of this, but with how Ventus is still important somehow, the introduction of Darkness as a character and how beloved Aqua is to the fanbase, I wouldn't rule out a bbs type of game in the future set in the realm of darkness. I wouldn't be surprised if after all they trio finds that there are worlds and people that are born and belong to the realm of darkness. Also now there are 4 new colleges to Eraques they probably didn't knew of.
In regards of Days, the only pausible way I see something it coming is if the future "DDD" v2 has Roxas tag along. There is still "the other one", and they made the mechanics for playable Roxas in remind so... wouldn't be surprised if that ends up being the case.