The whole discussion is fun in its own way to entertain ideas. But frankly, I think allowing people to truly believe someone else would have been warmaster is pretty ludicrus.But it's not much fun to say that the only person who could in any circumstance have been warmaster is, the warmaster. Where's the fun in that?
So perhaps we might discuss, if any of the other primarchs had been chosen, how would the Chaos Gods have tried to corrupt them? And would it have been harder for any of them?
Would for instance Vulkan, Sanguinius or Roboute have changed as quickly as Horus? I'd imagine Conrad, Perturabo, Angron and Lorgar deffecting the quickest.
I'm not a big fan of discussing Sanguinus. I think CotE hit the nail on the head when he explained how the Crusade would have demoralized him.
But then lets discuss the primarchs with good records. We have Dorn, Guilliman, and the Lion. I think an interesting idea would to think about Dorn. Dorn's confrontation with Curze while putting down a rebellion showed two different things in my eyes. Firstly it showed the warped way Curze looked at loyalty and the crusade. Secondly, I think it also showed Dorn's stern and defiant attitude towards that that went against the Emperor's Will, the Imperial Idea, and essentially... propaganda. For example, the closure of the Librarian order didn't mean the Emperor was going to abandon any use of sorcery or tampering with the Warp. When we see the Emperor's future plans with the Imperial Webway we can see that he was using that as a way of control over unpredicatable forces. Did Dorn see it the same way? Or did he look at sorcery as an Evil? It's been said that Dorn's mind is like a "fortress," "Impenatrable." (SP) Logically, he was good at keeping the Imperial Dream protected and defiantely close minded at accepting change. If the Emperor had been able to finish the Webway and then shown his sons a bit more about the plans of the future, at least Horus would have had an open mind to keep on following the Emperor without being surprised and/or disgusted. His meeting with the Interex displays Horus' open-mindedness to incorporate advancement for the future of the Imperium. So while Dorn basically became Warmaster during the rebellion of Horus, it can be said that it was only due to the circumstance of the Imperial State. There was a need for protection of the Strict Imperial Way. Any open-mindedness that would have expanded the state of being in the Imperium was thrown away. Dorn was never meant to expand the Imperium. Despite the current state of the Imperium, at least Guilliman was able to accept change and promote a new Imperium.
Guilliman is interesting because he represents change to the Imperium. Would he have made a good Warmaster? I think Guilliman represents dramatic change, and even as far as saying a flux of change. The state of the Imperium has gone constant change in order to fight it's different enemies, including itself at times. Though it has kept the Imperium alive, his dream of the Imperum was not the Emperor's Dream. Thats why I would say that he changed the Imperial Way the moment he made himself Ruler of the Imperium. He destroyed the legacy and capability of the Legions (bad or good).
The Lion is an interesting primarch. It would indeed be interesting to know what the Emperor thought of the Lion as secret as he was. If we looked at the Emperor in the way that he knows more about the Primarchs than they probably think he knows, then I would think there is a good reason why the Emperor did not appoint the Lion to that position. The fact that he lived on a tainted world signals a big no no for me. I believe it contributes to a few flaws that takes him off of being a potential candidate. The one motif that we see with the Lion throughout the heresy is his uncertainty with essentially everybody. He trusted the wrong people, and distrusted the right ones. On one hand he distrusts his legion and on the other he trusts Perturabo. On one hand it is hinted he knows there's something wrong with Luther after he attempted to kill the Lion... it even hints that he knew the attempt... and yet, he allowed him to live and prosper with recruitment. On the other hand he cuts of Namiel's head for holding himself with the Emperor's Edict. The amusing thing about all his actions is that there are justified explanations to account for them. Yet, it all leads to the destruction and dishonor of his legion. It seems like his actions are for the lack of better words... warped.