If you look at the Advent Calender main page it actually already confirms what each story is about, at least the faction that it is about. From today;
Day 9 - Flesh Tearers
Day 10 - Death Guard (HH)
Day 11 - Inquisition
Day 12 - Tau
Day 13 - Necrons
Day 14 - Imperial Fists
Day 15 - Thousand Sons
Day 16 - Marines Malevolent
Day 17 - Mechanicus
Day 18 - Blood Angels
Day 19 - Dark Angels
Day 20 - Imperial Fists
Day 21 - Raven Guard
Day 22 - White Scars
Day 23 - Gaunt's Ghosts
Day 24 - Inquisition
Day 25 - World Eaters
Just a theory here, but according to the blog entry that was made a week or so ago, the 25th release will be a full novel. Now this calendar could only chart short stories, and that would mean that a short story and a novel will be released on Christmas Day. But I think that the calendar is telling us that the novel is a World Eaters novel, and on top of that the blog entry said that the novel was focused on a character from the 40k universe that we all know...
A Kharn novel perhaps?? I truly truly hope so. :biggrin:
LotN