I am most interested in what Abaddon's personalty in the Black Legion Series will be. And if any of the Daemon Primarchs or other well know chaos champions like Erebus,Lucius,Khârn,Ahriman,Lufgt Huron,Kor Phaeron and others will appear.
Well, the lore says the Emperor's Children steal Horus' body and try to clone him. I assume that means we'll most likely see Lucius for sure. And cloning Horus? That seems to have Fabius all over it.
But really, there isn't much set lore during the period right after the HH. All we know is the BL courted each of the Chaos gods in turn and the other chaos legions attacked them. ADB is free to write anything he wants. Be interesting to see if any of the traitor primarchs/daemon primarchs show up. And dare ADB clear up what happened to Russ/Corax who were both said to have disappeared into the Eye? Highly doubtful...but fun to think about.