I think one of the things that most of the HH novel writers have done is create some internal conflict with all of the primarchs. Some of the primarchs already seem to be touched by Chaos before everyone took sides.
It's very easy to imagine Sanguinious becoming consumed by the black rage and turn into a full-on space vampire or how Russ could be a plundering space werewolf-viking and loot the universe. On the inverse, even the Night Lords novel gives Cruze some redeeming qualities.