I think Loyalist War Hounds / World Eatters would be a lot of fun. Still savage and fearsome, but loyal to the Emperor.   Iirc they had to purge 50%+ of their Legion because despite the nails, they were still loyal the the imperium. Even some of those who turned, including Khârn, did so for loyalty to their Primarch and not so much to the ideals of Horus.