Join Date: Aug 2009
Location: Portland, Oregon
Are we assuming Ullanor is still a success and none of the Primarchs are killed in its prosecution?
Someone needs to pick up the reins. Of those assembled, I think Horus still would have won out. Everything that had the Emperor select him as Warmaster is still exists: his winning personality, good relations with most of his brothers (or lack of enmity, anyway), excellent war record, and the prowess of his Legion.
The big change in the Crusade is that I bet Horus would have had softer policy on aliens and non-threatening human societies. After all, when he learned that the Interex was a combination of an alien and human society, he was willing to accept that.
It would have been interesting to see if, say, an Imperial force had decided to save a society with a better understanding of the Warp than humans at the time (say, an Eldar craftworld or something). In payment they exchange their knowledge of the Immaterium, thus preventing the Horus Heresy.
Maybe the Word Bearers wouldn't have gone so far for Chaos once the target of their hate and bitter feelings, the Emperor, was gone?
Then again, on the other hand, they may have had more success converting the Imperium. No one outside the WBs and the Emperor really knew how the Warp works. The Emperor would have stomped out any Chaos worship as soon as he saw it, but without him, the WBs would be more free to showcase the more tolerable pieces of Chaos worship. Before anyone knew something was amiss, it'd be too late.