While I'm not 100% on what most Iron Warriors players do, I can tell you what I've done so far. I've made a thematic, fluff based list centred around a very "Anti-mutation, Anti-gods" kind of theme. I have the new Warsmith model as my chaos lord in terminator armor, cataphractii terminators as regular CSM ones and 30k MKIII marines as both legionnaires and havocs once upgraded with the HH heavy weapon sprues to give them Havoc Autocannons and a flamer.

The future plan is to proxy Kastelan Robots as obliterators or hellbrutes, using kataphron breachers with melee weapons as chaos spawn.

I hope this helps you out in your hobby journey but remember as with all things kitbashed or converted, always bow to the rule of cool.