>>49918876
All album stories seem to be written to some degree around the songs in them. I suspect the next album will have 10 tracks, the Hifuu albums have always alternated between 11 and 10 tracks, and TJD had 11.
The tracks will also most likely be from 19 and 20, although it might also have some tracks from 16-18.5. I don't know what to make of the potential 16-19 track stories, but here are some ideas for some 20 tracks:
Sacred Forest/There's a Monster in the Woods - pretty obvious one, they go to a shrine's forest and find some creature that leads them along to further the story.
The Maiden Sinking into the Deep Sea of Memory - the title is pretty ambiguous, but it could work well for showing Maribel's dream adventures again. It's very LoLK-ish, so it could also work for them going to the Moon again.
Sacred Precincts of the Pyramid/Because I'm Used to Being the Last One Left ~ Stone Goddess - it could fit in a kind of NToJ2, there are a few pyramid-like structures around Japan, and Iwanagahime being an almost primordial deity, combined with the various archeological sites near Yatsugatake, and its connection to the Youkai Mountain, under which is located the world of the gods from NToJ. There's a lot you can do with this.
Fantastic Drift/Hallucination Atop a Bivalve - chasing after conspiracies is pretty stereotypical for the Hifuus, although that side is not shown enough imo. I'm not sure where the music of these two would fit in the album's overall movements though.
That being said, the stories sometimes relate to the track in more abstract ways, like track 6 from R35M.
Also, it will probably have at least 4 unique tracks, so it could go in a few ways.

Now, here's another possibility. MoF was largely skipped by the albums for some reason.
I think Maribel and Renko investigating the Suwa shrine and accidentally teleporting to the Moriya shrine would make for a good story. There's a lot of lore around the Moriya shrine that hasn't been fully explored, and Sanae makes everything she's in better. The way I specifically imagine such a story is that the two would be transported to the events of MoF, seeing the protag climbing up the mountain from afar, maybe talking to Sanae and Kanako a bit, or just spying on them. It would require a lot of landscape descriptions and subtilty in the dialog to not feel forced, but I trust that ZUN can make it.

If the story didn't have to stick to the game tracks, I wish it would be about time travel. Not necessarily about Maribel and Yukari's relation, more just about time travel in general. Something like them finding a physics manual somewhere which mentions time travel, they test to see if it works, weird stuff happens, they figure out what time travel model ZUN decides to follow, at the end of they send the book back in time for them from the past to find and read it.