Normally, people will slap on a bunch of chaos spikes and nonsense, and call it done. But, most of the time, you can tell thats all they did.
If you're up for a tiny bit more effort. I'd say take some plasticard and make "minor" alterations to the armor. Such as the "arrowed or pointed" banding running around the edges. Nothing too serious, though. Cause, if the chaos have one, its EXTREMELY old. So, making it look old and weathered is probably your best bet!
(Maybe even making a very "Tzeenchy Icon" section of the armor, for the thousand sons theme?)
But, even minor painting things can change the feel! Check out this example from a "2-second google hunt".
See how just the slight weathering effects and the emblem on the door ALREADY make it pretty intimidating, and less imperium?
Make sure to get rid of all the aquilas and termy iconography (if its got em), and make it look like its survived millennia of marauding man kind, and I think youre golden!