It's been a while since I revisited this army, but the release of the Tzeentch battletome has inspired me into adding more units. I liked the mention in the fluff that Tzaangor will brand chaos monsters like Cockatrice and Jabberslythe with their own marks (shame you can't do that in the game)
So to get me started: a Tzeentch Magister with his head swapped for a Tzaangor (the severed head that comes with the Darkoath Chieftain)
The hunched posture and the claw-feet work well with the new head (even if he does look a bit like a weedy Tzaangor or a dismounted Shaman). But it adds a bit more magic to the warband!