If you wanted to go with an Iron Warriors theme, then you should probably go with a warpsmith as your CSM HQ and a few Daemon Engines in your army like Forgefiends and Maulerfiends. If you wanted to be competitive, I would recommend a few Obliterators with the mark of nurgle. If you got those, you could theme you army as iron warriors/nurgle daemon alliance which could be interesting models-wise, and competitive games-wise. Daemon allies with a great unclean one and plaguebearers are good a keeping objectives and plague bearers can destroy light tanks with their plageswords. If the daemon prince is nurgle, thats good. If it isn't, then you should get one. Daemon princes with a 2+ cover save in ruins or with wings is very good. To make it even stronger give it a few psyker levels and roll on biomancy hoping for iron arm. Then for troops you already have plague marines, put them in a rhino and they become a decent unit. From their you can expand your army. Hope this helped.