Well new book to be announced by wotc tomorrow.
Likely ravenloft related if not a sourcebook for it as the the tagline "The mist beckons" is being thrown out there.
Honestly if you need source material just grab the 2e books and pick apropriatly themed monsters.
https://mobile.twitter.com/Wizards_DnD/status/1363896392306065413?s=19
Ougi Board saying The Mist Beckons Tweet.
I was RIGHT. Many folks said it wouldn't be Ravenloft because we already had Curse of Strahd, but CoS isn't a Campaign Setting Book like EgTW, GMGtR, MOoT, and E: RftLW (just like the SCAG isn't a Campaign Setting World Book for FR).
It just makes sense they are doing this, they want to focus alot more on Campaign Setting books because they sell well and they have DM and Player content in one book. So to double down on Campaign Setting books they have to really clear the decks first and that means addressing the weird limbo Ravenloft and the Forgotten Realms were in as the only DMGUILD legal settings, published settings with no 5e Campaign Setting World Books as support for published Products. This also gives them a chance to address woke whining about how certain groups are represented in settings they have committed serious resources too (they can't afford to discard FR and Ravenloft like they tried to with Dragonlance, 5e is too invested in these settings and publish material is already out there).
So publishing a Ravenloft setting and FR setting books clears the decks so they can do Spelljammer, Darksun, Planescape, Mystara, Dragonlance if Hasbro makes them, ect...
The two Undead/Spirits subclasses from the UA were Ravenloftian obviously. Same with the Dhampyr, Hexborn, and Reborn.
And I think the two Dragon themed subclasses from UA are for the Forgotten Realms. People say Dragonlance without looking at the subclasses fluff and asking if it fits Dragonlance or FR better (it fits FR better). Dragonlance has Dragon in the name, but FR has 10 times the Dragon lore, Dragon diversity, Dragon offshoots, Dragon Gods (it has a Pantheon of them), named Dragons and demi Dragons, Dracoliches, Dragon ruled Empires, Gem Dragons, Lung Dragons, Turtle Dragons, ect.... Hell the MtG symbol for the Forgotten Realms is a Dragon's head in a D20.
Dragons are as iconic for FR as Dragonlance, so yeah we are getting a FRCG type book this year, I'm doubling down on my prediction! Because I was right about Ravenloft!