I've never been overly keen on High Fantasy, as my tastes run more to the intersection of Dark Fantasy, Sword-and-Sorcery, and Historical. If someone would figure out how to write a Brother Cadfael / Cthuthlu crossover, I could die happy. But I digress...
What I feel Age of Sigmar is lacking is in reasons for intra-factional fighting. Now, some of the Alliances have plenty of reason to fight: Destruction is always ready for a brawl, Death has all sort of "palace intrigue" going on, and Chaos is... well, Chaotic. But how do we justify, narratively, two players' Stormcasts going at it? Why would Seraphom fight Seraphon? What can turn Aelf against Aelf?
I say we dust off a very old concept from the World That Was... The Gods of Law.
The gods of Law weren't "good guys," in the same sense as Sigmar, Morr, Ulric, or the regular pantheon of any Old World culture. They were, in their own way, just as dangerous as the Chaos Gods. Where the four main Chaos Gods represent constant dynamic change and possiblity, the Gods of Law represent unyielding, unchanging, perfect stasis.
Enter the Sons of Solkan.
So let's have Solkan come back and corrupt some Stormcasts, Devouted, and Seraphon. Whispering promises of retribution against Chaos, punishing the oathbreakers of Death, and bringing civilization to the barbarians of Deatruction... and dammit of those other Stormcast aren't going to help, maybe we need to show them the error of their ways. With a hammer. To the face.