I've seen one where the Emperor joins Chaos, making Horus and his traitors the good guys and radically altering the storyline even more grimdark than normal WH40k to a significant degree. The Dornian Heresy is not alone - there are many other alternate Heresies that I've seen as well. It's very well thought out and radically different while still easily recognizable. There's still a Drop Site Massacre, Prospero is still razed by the Space Wolves, the Arch-Traitor (though it's Dorn this time) still ends up obliterated by the Emperor, the Emperor still ends up mortally wounded on the Golden Throne (though the warp rift in the palace is permanently sealed by Ahriman, seemingly at the cost of his own life but it's rumored that he's secretly still alive and trying to find the Black Library on Magnus' orders), and there's still an Ecclesiarchy formed almost immediately after the dust settles though it begins life directly under Lorgar. Other than that, much of the general plotline of the Heresy is remarkably similar.

Many of the Primarchs' backstories are altered so as to make them more predisposed to stay loyal or join Chaos most radically changed is Angron, which causes the World Eaters to become noble but fearsome paladins as opposed to bloodthirsty berzerkers. Rogal Dorn becomes the Arch-Traitor, and the loyalist legions stay as whole legions as in this timeline it was them being spread out too far that allowed heresy to take root. Ever heard of the Dornian Heresy? It's an alternate timeline for WH40k where the Primarchs' loyalties are all switched around during the Horus Heresy, save for the Ultramarines (kind of) who leave the Imperium but don't join Chaos, instead forming their own independent realm.

Horus heresy timeline