They've settled into a "big story" and "small story" rhythm. One tied into videogames and minis and more. And one that's not. The small spring story that is mostly a reprint (since it's coming so shortly after the fall book) and the big summer story that details the Realms and acts as a gazetteer.
Looking at classic adventures:
- Desert of Desolation is a good choice. Combining three adventures, it should be pretty large. (128-pages apparently. So they only need to expand it by 100 or so. Which is easily done by having some more setting description and low level adventuring.) Plus, deserts, mummies, and ancient tombs (that are actual tombs) haven't been done yet, and are classical but slightly different.
- Gates of Firestorm Peak is a biggie that hasn't been touched. That could work. Get into the Far Realms lore and touch on aberrants. Would need a lot more expansion.
- Dragon Mountain and Red Hand of Doom likely feature monsters that have been in a spotlight too much. Dragons need a break. Ditto the drow and Queen of the Spiders... although they did overlap giants and Acererak. So that's no guarantee.
- Scourge of the Slavelords. This one can easily fill a book.
- Undermountain. Which was teased and set-up with Tales from the Yawning Portal and is a pretty iconic mega-dungeon. We've seen a lot of small dungeons, but no singular megadungeons.
- Rod of Seven Parts. If this was planned, it might not have been a throw-away element of TStorm King's Thunder. But… it did tease that item and set it up for a future story…
- Dancing Hut of Baba Yaga. It's been commented in the past, that this could work well as a feywild adventure. I'd love a faerie adventure.
Looking at big villain types that haven't been featured:
- Githyanki. Iconic to D&D, having been featured on the cover of the Fiend Folio. And invasion of the material plane ending in a planar jaunt to the Astral Plane and siege of a Githyanki city would be cool. But… it's not evoking a classic adventure, so WotC likely won't do it… (Although, Chris Perkins did write the high level Lich Queen's Beloved for Dungeon Magazine.)
- Mind Flayers. These were skipped in Out of the Abyss. Likely so they could be used later. Again, this could tie nicely into Gates of Firestorm Peak. I imagine they might wait until 2019, and tie it into a psionic book. Or something with the Mind Flayers and githyanki fighting. But, again, it's not evoking a classic adventure, so WotC likely won't do it...
- Orcs. There's not really any iconic orc adventures, so these guys have fallen to the wayside… And probably will continue to just be random encounters. But since WotC is only interested in call-backs...
- Devils. These saw a little attention in Tyranny of Dragons but we haven't seen Asmodeus really make himself known. And, one more time, no classic to draw from. Well… maybe A Paladin in Hell...
- Genies. Likely to Al-Qadim. But I'm gagging for a big adventure that goes to the City of Brass, which has always been a criminally underused part of D&D, despite once being featured on the cover of the freakin' DMG.
- Fairies/ fey. See above.
Spring 2018
I think
Scourge of the Slavelords is high on the list of most likely updates. It can be given the
Curse of Strahd treatment and made a little more cohesive, while also working as a series of smaller adventures. Because people want smaller adventures.
Summer 2018
Then maybe a reimagined
Desert of Desolation, placed in Mulhorand.
(Either could be replaced for
Undermountain.)
Fall 2018
A book of magic items. Maybe with some lore on crafting and shopping, with some magic item shops. More artifacts and lots of advice on creating artifacts and building campaigns around them.
Ideal title:
Bazaars of the City of Brass. Detail the city and its markets.
Maybe also the artificer, now that I think of it.
Spring 2019
Time to get weird. Expanded
Expedition to the Barrier Peaks
Summer 2019
Summer of aberrations Gates of Firestorm Peak mashed with invading Githyanki versus Mind Flayers.
Fall 2019
Psionics and the Mystic. Maybe something on the planes. Planar gazetteer, like the Sword Coast Adventurer's Guide.