The D&D team seems to do two high-energy books a year, typically an adventure and an accessory. Because it takes a long time to write and develop an adventure: Chris Perkins can't do two a year. And so there's often a low-energy repring in the Spring following the big summer storyline adventure and the fall accessory (i.e. Curse of Strahd, Yawning Portal, Ghosts of Saltmarsh).
Of course, there's always exceptions. Like when they swapped the place of the fall and spring products to have back-to-back Waterdeep adventures.
Now they seem to be padding the product line with a fourth partnered book, which can also be a little low energy. Ravnica, Acquisitions Incorporated, and Theros. With the MtG books done by some of that team, using recycled art. And, of course, the Critical Role as a low-energy book replacing the spring low-energy adventure reprint.
For the rest of the year, I expect two books. Really, this is the easy prediction: there was a lot of push-back from having three fall books in 2018, so I don't expect they'll do that again (AI was in the late spring for a reason). And they've done four Starter Sets now, so I don't expect a fifth. That market seems well saturated.
So we'll get the summer storyline adventure and the fall accessory housing the subclasses we've been testing. There's no way they wouldn't do a big storyline adventure: it drives their product release strategy. And they have to be testing all the UA books for something, and they're being released roughly in the same timeframe as content was for the Guide to Everything.
What is the adventure going to be? shrug It could literally be anything. There's a dozen options.
They haven't done abberrations/ mind flayers yet, and they're a popular villain group. So that's an option. And ties into Guide to Monsters and potentially the githyanki entry of Tome of Foes. As well as the centerpiece of the last mini set. And the new Baldur's Gate game. So that's the easy bet.
But they also haven't done an orc heavy encounter. Or something with kobolds at the forefront, such as an updated Dragon Mountain. Genies in Al Qadim and/or the City of Brass. Fomorians in the Feywild. Rakshasa.
The accessory is easier to guess: it will be Noun's Something of Something.
Literally any random proper name from D&D partnered with a vague description of contents.
Elminster's Librum of Wonder. Drizzt's Lexicon of the Exotic. Zagyg's Dictionary of Depravities. Whatever. The title will likely have very little tie to the contents.
I expect sublcasses and... more rando content. Whatever they feel like slapping in a book. Advice on homebrewing. Optional rules. Pancake recipes.
They filled Guide to Everything with tables of random names, encounters, background generation, and other oddities. There's no telling what they'll put in the next expansion book.
I doubt it will be a campaign setting. By the time we get Theros we'll have seen three campaign settings in a row. I can't imagine WotC doing four campaign settings in 13 months.
While I expect we'll see another MtG book in 2022 (because, again, low energy book mostly written by someone other than the D&D team) I don't foresee many campaign setting books from now on: that small niche seems well filled. We have five setting books for 5e, and that's probably three more than any group will use.
The actual interesting question is: what will the fourth book in 2021 be? I think we can expect another collection of reprint adventures and some fall rules expansion (maybe more monsters). And the summer adventure so the partners can include more content in Neverwinter and make miniatures sets. But it's anyone's guess what the fourth, likely licenced and freelancer outsourced product will be...
