Starting off with the obvious (December 10th):
Festival of Yay
Festival of Yay Seasonal Snacks
Festival of Yay Wubbox Costume
Festival of Yay Jr.
Adult Yool on Wanderer
Shadow Islet
Now for the less obvious (December 10th):
Fandhul as the Cold Primordial
Control/Ruin Double that plays the Harpsichord
Epic Floot Fly that is either blue or purple
Epic Cranchee revealed in the trailer but not available
Epic Scargo teased on the loading screen and added, but not available
I like Fandhul as the Cold Primordial because of the already established cost order of the Primordials.
While I would like a Depths Double, Control/Ruin is more likely to me.
Faerie Island first released in December, it would be fitting if Epic Floot Fly also did.
December 17th:
Epic Cranchee made available
Epic Scargo made available
Epic Cranchee is the Cold Mythical's Dreamythical, need I say more?
Scargo variants have always released in Mid-December and I don't see a reason why that should stop with Epic Scargo.
For my final, and wildest prediction yet (December 24th):
Faesoddian Foxglove
It would be wrong if Faerie Island's Titansoul didn't release during the Island's month of origin. I also wouldn't see them pushing it all the way to Cloverspell, maybe Crescendo Moon, but Christmas Eve works better. Faesodd's Titansoul can be the Christmas present for the community to top off one of the biggest years in MSM history.
Epilogue (December 31st):
Rare Teeter-Tauter
Greeny's arguments for the Rare Workshopper timeline are too good to ignore.