Chaos also has a large depth to its collection though, with the number of different unit types (and the whole Trator Guard via IA13..)That's something to think about. GK ally naturally with any space marine army, Astra Militarum, and Inquisition. Chaos tends to like... chaos, so Daemons and CSM ally well.
I still am in favor of chaos but branching out from your start point isn't a bad thing to think about. But don't overwhelm yourself like I did when I began, haha.