few quick comments, if I may

1. banner - I've grown quite a big personal hate towards freehanded banners (clothes, eldar falcons etc.). Maybe it's just me, but I can't imagine skeletons painting sunsets over castles on cliffs...eldars airbrushing naked ladies over their machines of war...or horrors painting images of their lord...
yes, you can also call it envy, as I can't paint like this

apart from this subjective comment, I think that blue flame on the standard could get slightly brighter final highlights.
2. horror - lovely skin on the first pic, with rich colours, great contrast and smoothness. but, transitions look more abrupt on the second pic. not sure, maybe it's a matter of photo angle, maybe it's WIP.
3. sorcerer - not much to comment so far, but I like parts which I think are finished (or close to that). I think I'm gonna love red glowing eyes here.
4. bases - not sure... tzeentch is the master of change, mutation, and I tend to think of fire when it comes to tzeentch (there still are these fiery tzeentch demons, right?)
so, maybe some small warped trees/bushes? or simple ground/grass/slate/ruined slabs with blue flames (just as on the banner)?