Following up on Marco and Seaglen's comments, I'd go one of two ways:
If created by a chaos mage, go with some Khornate branding and barbed wire. I think the Khornate bunny ears are unnecessary, a simple scar or brand sculpted into the chest would do the trick. I also quite like the idea of giving him some kind of barbed wire attack (a la the final scene in the Silent Hill film) just for some variety. A complete berserker would be tempting, but given the strength of the game is character there could be a lot of possibilities with having a character that can do more than whack you with their boney appendages.
If created by an Inquisitor, Marco's suggestions sound great. I'm 50/50 on whether the barbed wire would still work (holy bared wire? sanctified razor wire?), maybe some tight chaining? That could be hard to do well, though I did see a great tutorial on making GS chains recently, though it looked very time intensive. Injectors w/ purified blood, holy water, etc to keep him sedated would be good too. If you wanted to go over board, having a servo skull (or two) that follows him around, containing the daemon in a hex field until it's time for him to be unleashed could be interesting.
Anyway, great looking model and concept! If you need any proof reading, extra model shots, etc let me know. I have a few radical models that need revamping and this could be a good push for me to do it.