I imagine that such a hypothetical "Chaos Champions" series would work better as a single book: an anthology similar to Treacheries of the Space Marines but entirely focused on the special characters of Chaos (perhaps not even limited to the 4 champions! Huron in the Palace of Thorns; an Abaddon short by AD-B as he renames the shattered Sons of Horus as the Black Legion; perhaps stories from Fabius and/or Cypher).
Certain famous events of these characters have already been depicted, though:
I also imagine that James Swallow's next Death Guard novel will cover Typhon marooning his fleet in the warp, prey to the Destroyer Hive.
A short story for each of these characters that touches on their famous events would be welcome, but I would want the stories to cast new light on the event, or have some unique angle that they cover. I don't want a lazy rehashing of what we've already seen or will see in the Heresy books to come.
I for one welcome more Ahriman fiction! Which reminds me, I still very much have to get my hands on a copy of Atlas Infernal
(yes Xisor I know it's fantastic and I trust your judgment of 40k novels implicitly* you don't have to blare at me to buy it)
*though I still want to take a look at Brothers of the Snake to come to my own conclusions. It sounds like something in the area of my interests, but also my reading taste has pretty closely copied yours, so...