I can't believe I missed this news.
Didn't Nomura say that Square Enix plans on releasing a new Kingdom Hearts title each year? Assuming ReMind counts as a seperate title, I assumed this under-development title is going to be a 2021 release.
I have a gut feeling this is the X HD remake. Union Cross has too much canonical value to be stuck on a server dependant mobile game.
Like everyone else has already said, that was the plan leading up to KH3 so we had stuff to do. Now that the game's out, the only thing we really have to look forward to is ReMIND and Union X's story updates. Once the DLC releases we'll have an idea of where the story is gonna head next, but UX is going to be where the attention goes from here on until the next HD title comes out.
In an interview, Nomura mentioned something about a "double feature", so it very well could be a Union X title for PS4 and Xbox One, FFXIV-style. It would make more sense, since even though mobile games are a thing in Japan, it would be a good way to keep the franchise alive for quite some time until the next game comes out.
In fact, maybe that's why we ended up getting the Back Cover movie, and why the Foretellers even appeared in the Epilogue -- the assets were already made so they can just use that for a potential UX console game, a lot of Daybreak Town is already created. Plus, it being on HD consoles allow the story to expand in ways they probably can't do with a phone. I'd like to see it.
Either way though, I really wonder what the "double feature" could be.