Big squad was my thought too. Kinda why I was thinking World Eaters for that extra attack, and even if they need to drop in front of enemy lines because of space issues, they can still hurt. And now that I'm full of coffee and thinking straight, they could use the Veterans of the Long War stratagem too, whereas my Sons of Malice couldn't.
I would make Iron Warriors Warp Talons just to call them Iron Butterflies, with custom mech wings
The Daemons book is apparently imminent so perhaps the Heralds will have something to share with their mortal wannabes.