I just read this on Aaron Dembski-Bowden's blog.
So let’s just say that he came back to Earth and changed his mind. We’ll leave it at that. His next Horus Heresy book will probably be a Night Lords book, set in the Age of Darkness period – the several years after the Dropsite Massacres and before the Siege of Terra.
I haven't read Soul Hunter yet (will get to that soon), but this sounds interesting.