I swear, there has been a LOT of contradictory hints all over the place; I'm going to try and organize some of them, from serious hints to more vague.
- Gem Dragons being in a book this year (this one I find very likely, the Sapphire dragons are already done and released, and was done months ago)
- Welch's "Adventure Compilation" project (including writers Marisha Ray and Debora Ann Woll). This was mentioned months ago, and although it is possible that it was quietly cancelled, I find that possibility strange considering Welch was even willing to talk about the project.
- There is so much UA material that isn't going into Theros (I'm not certain, but everything post-October isn't in Theros). Where does this all go? What is it for? Psionics, new subclasses, class variants, spell tattoos... it's a lot of material and has no clear location.
- Perkin's "Gazetteer Towns" hint
- Perkin's hints of "If you love Curse of Strahd, you'll love this!" Likely the adventure (as that is MO) but could be anything from just a scary-tone to full on Ravenloft return.
- WotC Mindflayer/Spelljammer hints. There are a couple here and there hinting that they want/are doing something with Mind Flayers.
- Perkin's hints of Lantan (I do not remember the source here), but it was that the Lantan tech in Dragon Heist was just a precursor to future material there.
These hints are a mess, and the only clear way to sort them is into 3 books (and I'm not confident in this theory);
1. Welch's book (no idea the theme or what this is; honestly the gazetteers could fit here if they are adventures all over FR).
2. The annual adventure book, likely Lantan plus Spelljammer or Planescape.
3. A monster/player's book, combining new player options with new monsters like the Gem Dragons. Possibly Planescape themed.
This is my broad theory for the year, even though three new books in the fall/winter is a lot (though not unheard of; looking at you 2018).
Now that I've thought about it more, Perkin's gazetteers make sense in Welch's adventure compilation, especially if it was scattered all over the Forgotten Realms.