Suprise this has not been posted, the Black Library Calender, from a soon to be release Heresy novelhttp://www.blacklibrary.com/advent-calendar
If I was a guessing person. I'd guess this is Iron Warriors vs Imperial Fist.
Door No2 - Show yellow & black strip.
Door No 14 - This show the strip on the weapon
Door No1 - It yellow Power Armour. Look like the bording Marines that Forge World release a while back & Games Day only model. Door No16 should make this clear
I think this might be base on Graham McNeill forth coming novels which deal with the Iron Warriors & Emperor Children. If you check out his video interview over on the Black Library web site.