I would free up some points by dropping the havoc launchers and the extra armor on all the vehicles, especially the rhino's. Rhino's are meant to be cheap, and the more points you invest in a transport, the less points you have for the real killy stuff. I would also drop the melta bombs on the plague marine champ. He has a powerfist, and the squad has 2 meltas, that should be more than enough to take out armor.
If you're going to take a tzeentch demon prince, then you may as well use one of its biggest advantages and take 2 powers. My favorite load out for a tzeentch prince, is warptime (which you have) and wind of chaos. Cast warptime at the beginning of the turn and now you have a re-rollable flame template that inflicts instant wounds on 4+, and you can still have the charge. In order to make this happen, you'll have to free up some points.
I'm guessing that you like t sons, so i'm not going to tell you to drop them, as they do have their uses. One critique though are the sorcerors. They have wind of chaos, which is a template power, the last thing you want is your thousand sons to be close enough to use a template, because if they are, it means they're close enough to be assaulted. Sure they have good enough staying power, but their use is with their ap3 bolters. I would suggest the power (can't remember the name) that is an 18" assault 3 ap3 shooting attack, this way you get a few more ap3 shots and you're out of assault range
Your plague marine load out is perfect, once again, just drop the melta bombs
Your heavy support looks good. Just one word of advice, 2 oblits are far better than a lone defiler. Obliterators can be meq/teq, anti armor, can get cover saves and are far less likely to be pointed out due to their smaller size.
With a few small changes i think your list could be quite deadly. I hope this helped and good luck
EDIT: curse you chaosveteran
, it seem's i am too slow of a typer