If the Scars had gone Traitor...

[Rune Priest Ridcully]

I could see them falling to Tzeentch after reading scars and seeing the influence of the Stormseers and the various shamanic practices, or Slaneesh, though I doubt it would be in the sexual or decedant manner of the emperor's children, possibly a far more insidious form, like a sort of Goddess who rewards those who show speed and skill in battle, a goddess who whilst asking them to bath in their emotions and sensation, to not lose themselves in it, as after all, excess itself becomes banal and dull if it is all mortals perform for you.

[Dam13n]

One could make an argument for (almost) any of the gods.  Khorne, due to the more feral aspects of their nature.  Slaanesh, due to their love of speed (not all excess needs to be sordid).  Tzeentch, due to their mystical nature and the regard they hold their Stormseers in.  Nurgle...

 

Well I actually struggle to find a good reason for them to fall to Nurgle.  There doesn't seem to be a convincing link, personality wise, between Nurgle and the Scars.  Of course all it takes is a (not so) healthy dose of Nurgle's Rot and they may have found their hand forced into servitude.

 

In my opinion though, I think they'd have been more likely to become an undivided Chaos Legion, had they fallen.  Though I equally suspect that they would have fractured soon after the Heresy, which may well have spawned Khornate, Slaaneshy and Tzeentchian warbands alongside a plethora of undivided ones.

 

It's all supposition though.  They didn't fall, so we'll never know exactly what GW would have turned them into had they fallen.

[Carrack]

Nurgle because of the steppes of Chogoris.

 

Life on the steppes is a constant test of endurance. The herd has to eat or the tribe will starve. This means that the herd has to move almost constantly. Too sick to get out of bed, toughen up, the herd has to find new grass. Your back is a board of pain and stiffness from days in the saddle, and your legs are permanently bowl legged and chaffed, too bad, the herd has to eat. The frigid wind is too cutting to leave the fireside, or the baking sun has your throat parched, well the herd has to move anyway. Only the strong survive the ceaseless journey across the harsh steppe. Even the horses, barely bigger than ponies, can run longer than any other, and withstand extreme temperatures better than any other breed. Nurgle sees this and smiles on the people of the steppe, for the conditions are ripe for its people to call on him for succor.

[bento-of-the-earth]

Oh Nurgle definitely works. After all the IRL Mongols were the first army to employ biological warfare; on the walls of the Black Sea, the mobile cavalry army was halted so they turned to launching the corpses of their dead up at the walls. These corpses just so happened to have Bubonic Plague building up inside them, and thus a third of Europe bit the dust

Plus there's the part about their armor being so sturdy when tied up that they often never removed it, like not to use the bathroom or dress wounds...you see where that could be going...

I see them less as the bloated, hulking servants of Nurgle but definitely rotted, genocidal and spreading their contagions much like the Purge or other Forge World nurglite chapters. That would make another good reason for bikes and mobility in their force too: letting any pox stay stagnant is only going to have it die out, there needs to be new prey, new carriers for the death to spread on.