Players were kicking ideas around of alternate 40k history, wanted to hear other takes on what would happen. These are the scenarios regarding the Horus Heresy era treachery. What do you think would happen for the various alternate histories.
Loyalists arrive early:
The loyal Space Marine chapters arrive days earlier, forcing Horus to abandon his siege. The Black Legion flees leaving the Death Guard, World Eaters, Thousand Sons and Emperor's Children to be slaughtered. The warmaster survives the Heresy though his forces are much reduced.
Horus stays loyal:
The attempt to turn Horus fails utterly and Chaos turns one of the more unstable Primarchs that can't muster the same levels of treachery, leading to a much reduced assault on Terra.
Mechanicus threat:
Instead of targeting the Primarchs for corruption the Fabricator General turns, leading the massive numbers of Techguard facing the full number of Space Marines legions.
Horus wins:
Just like it says, Horus takes over Terra and the loyalist chapters must strike back from hiding.
Primarchs stay loyal:
There is no Heresy, Horus remains loyal as do the rest of the Primarchs, the Great Crusade continues unabated.
Mutual destruction:
The marine chapters destroy themselves though the Emperor remains, no loyalist or traitors can field any numbers that matter.