i know this sounds crazy but no one hates the chaos gods more than they hate each other. Why not add a chaos champion into the fray that aligns himself with the other 5 heroes. His careers could reflect the other gods as well. Such as Khore, slaanesh, tzeentch and undivided.
The character could be similar to archaon in the fact he was chosen by the gods(not nurgle of course) and goes on a rampage/crusade to ruin papa nurgles plans and slaughter the skaven. i Know it’s a long shot but i can imagine it would be very fun.
Thanks for reading