That's possible but barring a massive change in circumstances why might Horus not be Warmaster?
Ok, now that I can respond properly...
Massive changes in circumstances
are happening. Kesar was found around 900.M30 according to Daemon Hunter in rolz (I asked about it). In the span of twelve years, we've changed a lot. Alpharius is aware of Chaos and working with Malcador to utterly wreck the shit out of Tzeentch's plans. Konrad Curze has actual hope that things might get better. The Imperium has a dedicated anti-Chaos Legion. We've got a new religion brewing which might become dominant in the Imperium while not saying the Emperor is a god. The Imperium has accepted a race of xenos. The Fabricator-General is a lot happier because there's a Primarch that's sharing his knowledge rather than hoarding it, and some other Primarchs like Ferrus Manus are considering following his example. And a lot of other stuff.
Most pertinent is that Lorgar didn't go on his pilgrimage into the Eye of Terror until 967.M30 in canon, so he's decades ahead of schedule. What's more, with Lion El'Johnson royally pissed off at him Lorgar is unable to operate normally, and won't be able to do so even when he comes out of the Eye of Terror. None of the other Primarchs are siding with him against Kesar. With the Chaos Gods having much of their schemes wrecked, I think Lorgar is going to do something desperate. My guess is that he will try to attack Valhalla with the aims of drawing out Kesar to kill him, thinking that he might stand a chance having gained the blessings of the Chaos Gods. (he might even still think the Emperor is a god, given the Emperor hasn't rejected him and the Chaos Gods may play into his delusions to get him to cooperate) My thinking is that Lorgar has a good chance of fleeing, and becoming a persistent problem - he might take his Legion into the Eye of Terror for shelter (or maybe the Maelstrom, since that's closer to Valhalla and might be easier/safer to reach), or otherwise go to the edge of the galaxy to build up a Chaos Empire, or in some way become a persistent threat to the Imperium. Thus would begin the Aurelian Heresy, where other Primarchs who fall might gradually join him, or perhaps just go fuck off to build their own Chaos empires, or whatever.
Regardless of the specifics, with one of the Primarchs having openly fallen to Chaos and turning traitor, the Emperor's plans could radically change. Knowing that his sons are not beyond the schemes of the Chaos Gods, he might decide that the Imperial Webway project needs to be put on the backburner so that he can be more active in managing his sons and the Great Crusade to prevent more traitors, making a Warmaster unnecessary. Or he might decide that declaring any individual one of them Warmaster might be too risky, because Chaos would inevitably focus its efforts into corrupting that one. Or he might fear that and still need a Warmaster, so decide not to risk his favorite son Horus by putting him in that position. Or maybe Horus might just end up getting his ass killed by another Traitor Primarch, necessitating someone else taking the position. Or maybe one of the other viable candidates impresses the Emperor enough that they get the job. Or just something else I haven't thought of.
