Well from various sources we have some insight as to what would have happened if Horus had killed the Emperor on the fateful day.
However, if you read Legion you'll know that the last shreds of Lupercal would cause him to become regretful and bitter and lead him to destroy chaos through destroying humanity and his allies, feeding chaos until it consumes itself, making the warp a serene place once more.
says, the Xenos Cabal prophesied that if Horus triumphed over the Emperor, he would wipe out the Human Race in his guilt, even his closest allies would have warred against him in one final Chaotic apocalypse in which Humanity would become extinct and Chaos along with it. Take into account though that the Cabal were/are not infallible, and this wouldn't have necessarily happened.
Alongside this, Codex: Chaos Daemons
states that the Emperor is the only single entity preventing a complete Chaos victory, that he is shielding humanity from the worst depridations of Chaos, and without him humanity would by default be helpless before the dark temptations of Chaos and fall into its grasp.
As well as that, the Imperial Webway would be unable to be sealed and Terra and the Sol System would be flooded with Daemons and any remnants of loyalist Imperials (although its unlikely there would be any, given that the vast majority if not all of humanity would have fallen to Chaos) would be unable to utilise Warp Travel without the Emperor's Astronomican.
So essentially, if Horus would have triumphed all of humanity would have fallen to Chaos and Horus would have ruled over the ashes of the Galaxy. Chaos would be ascendant and nothing would be able to challenge its dominance.
but I dont think Chaos could endure are a unity without enemies like the Imperium. Unfortunately when the Imperium dies (and as the fluff shows it, it will sooner or later), Chaos will start picking on itself and warlords will turn upon each other to be the stronger. Then other aliens and robots will kill them as well.
Remember that Chaos is its own enemy as well, the Great Game is the obvious example of this. Chaos constantly fights itself as it is, but that doesn't mean it is weak. Without the Emperor, Chaos would be utterly dominant of the Galaxy as well as the Warp, no threat like the Tyranids or Necrons would have been able to challenge a Galaxy dominated by Chaos.
i honestly believe that abbadon has his own agenda for mankind than the chaos gods invisionment. there is something that tells me he wants to rule mankind without the darkgods, the reason i think this is because he will not ascend to daemonhood when he could have.
The Chaos Astartes codex specifically states that Abaddon wishes to forge an Empire of Chaos
over the ashes of the Imperium. He is too far enthralled to the Chaos Gods to ever stand a chance of ruling without them, even if he wanted to.
The reason he hasn't ascended to Daemonhood is because firstly he is much more useful to the Chaos Gods as a mortal and thus able to constantly lead wars and incursions into the Imperium, which he wouldn't be able to do as easily as a Daemon Prince (thats assuming he hasn't actually been offered Daemonhood). Secondly is the option that Abaddon has rejected Daemonhood for himself, because he knows that as a Daemon not only would it be harder for him to manifest in the Material Universe, but he would likely lose his current purposes and hunger for revenge.