Often, a simple head-swap can do wonders for the feel and aesthetic of a miniature. I have a small number of guardsmen figures that I'm creating with a "mutated" feel to them just as a mini-side-project to keep me distracted. The first head-swap effort was using a dryad tree head and I think that came out reasonably well.
This guy, instead of having an alien styled or totally mutated head simply has a helmet (from the chaos knights range). Sure, its not a massive conversion. But, I think it gives off an air of "sinister" and "chaos" nonetheless. I intend to paint this one up in the same colours as the previous guardsman I painted so that they can form a small killteam unit of Tzeentch renegades perhaps.