Thousand Sons and Sisters of Battle are on everyone's minds when it comes to December releases from Games Workshop. We have heard previous rumors revealing a big thousand sons release followed by hints at the end of the month of a Sisters of Battle Cannoness.
These rumors are what is being referenced from this source.
http://natfka.blogspot.com/2016/11/warhammer-40000-releases-for-december.html
Daemon Primarch Magnus
Plastic 40k Ahriman
Plastic Rubric marines
Plastic Rubric terminators
40k Tzaangors (bizarre but asked twice regarding this)
Sorcerer Cabal (Not sure if new models or re-packaging)
and at the end of december; Plastic Canoness for Sisters of Battle.....
The books are supposedly War Zone Fenris 2
Traitor Legions (not known if a rules book, an art book)
supposedly 8 mega force bundles in december between 40k and AOS (even split)
via anonymous sources on Faeit 212
Tzeentch/Rubricae:
The info you already have is spot on. Arrives with/alongside Warzone Fenris 2. Soon.
Sisters info: It's not just a Cannoness.