Necro-ing this thread, because we have our Q1 book (Candlekeep Mysteries), and a new UA for some Dark Fantasy races!
I still think we will be getting 3 more books this year, one per quarter. So here's my guesses;
Q2: Monster Book (probably dragon theme).
I still think this is happening, because gem dragons should be coming sometime, what with WotC making the Sapphire Dragon and hinting at more when they did. Plus, the UA from last October had some dragon-themed subclasses. A monster book is also very overdue, as the last was MToF, which came out I believe two years ago? This hits a few boxes so feels like it's definitely happening this year.
Q3: Annual Adventure Book
Every year we have gotten a big adventure, and that pattern probably won't break this year. Almost certainly will be in the Forgotten Realms, as they always are, but WotC seems interested in delving in FR locations we haven't done a deep dive in 5E. Where that would be for 2021, I don't really know. Gun to my head, there were some hints to Lantan and Spelljammer, so maybe that, but it could just be somewhere else entirely.
Q4: Setting Book
WotC has said in their streams recently that setting books sell well, and that they want to make more. Last year they released two (although Wildemount was made mostly by non-WotC people), so I feel it's a sure bet another is coming this year. What it was going to be I wasn't sure, between Planescape, Dark Sun, or Ravenloft, but the UA seems to pointing to a Dark Fantasy setting. Although a new Innistrad set is releasing at a similar time, I actually feel Ravenloft is probably getting top billing (although I wouldn't be surprised if Innistrad got a section, or was at minimum mentioned, in such a book). A MTG setting was released last year, and I think WotC will alternate to a classic setting this year (and they've said they are working on releasing 3 classic settings). I'm sure the two brands will be doing cross-promotion on the horror products, even if they aren't the same settings.