That said, using the Yawning Portal as a framing device for a bunch of unrelated dungeons does feel like a missed opportunity. One can't help but wonder if they were originally going to do an Undermountain book and then changed their plans but decided to still make use of the intro they'd cooked up.
It's a bit rich of them to say, "Hey look, here's an inn with this super-interesting entrance to a mega-dungeon it. We're going to tell you all about the inn's proprietor, and the fact that he charges a gold coin to let people go down into the dungeon, but we're not actually going to let you go down there. You're just here to pick up the hooks for some other dungeons that are far away."
Well, it certainly seems that Perkins got slammed hard by having to do Curse of Strahd and Sky King's Thunder back-to-back, in addition to the decision that they didn't need to release so much adventure material ever year. Wouldn't shock me if you are right about what went down.
It'll be interesting to see what books they release in the next couple of years. Just in terms of adventures, it seems they have heavily seeded a couple in particular:
- Great Modron March keeps getting brought up in the books, lots and lots of seeds planted: serious Modron obsession in 5E, and a great opportunity for Planar exploration
- Gates of Firestorm Peak gets namechecked in both the PHB and DMG, and Volo's has some really intriguing hooks for Mindflayers: everyone loves Cthulu, right?
- Demons got their big show in OotA, maybe Devil's will get their time (maybe also in a Great Modron March, who knows?)
- Undermountain is getting mentioned a lot, don't forget the Xanathar is also in Volo's Guide Beholder chapter and the cover of the Monster Manual: probably movie related plans.
- Genies, and Genie politics, get some fair amount of attention in the MM. Would expect the City of Brass to end up related to something, somewhere along the way.
- Deep sea adventuring, with details of underwater societies and ruins, would seem plausible: maybe a bit of return for the Kraken Society
- Something Goblinoid related, a war campaign perhaps, bringing in a lot of the hooks from Volo's
Any other plausible candidates?