Radiant Citadel was awesome, one of the most original products WotC put out in years. My favorite adventure anthology. Can't wait to see if they do more with it!
They mentioned that card on their Discord:
"Lastly, a little holiday gift from us: a D&D-themed holiday card featuring art by Natalie Andrewson, showing an adventuring party making a rest stop at a warm castle on their long journey with presents and goodies in tow. Here’s to warm rolls, cold...
It's cute. The species are actually kind of interesting (I could see myself playing the fox or the snowman), and there's some cool monsters (a snow sphinx and a pine tree drake).
Yeah, pretty easy guesses, mostly. The PHB mentions the World Tree in the same vein as other methods of traveling worlds, so I thought it was the next logical one after Infinite Staircase. It could be adventures based around trees. Which would be neat. And by "Everything dropped" I think they...
My answer is amended to:
Ravenloft Players Guide
Dark Sun Players Guide
Dark Sun DM Book
World Tree Adventure Anthology
A splat book, with "Everything" dropped