Join Date: Sep 2010
Location: In the shadows ...
I personally see it falling upon Dorn or The Lion. If Guilliman had died on Calth then Dorn and El'Jonson might not have made their decisions to travel to the Iron Cage and Caliban respectively -- or at least not in the same manner or at the same time they did.
Likely the Lion, if he took command, would have sent some of his legionnaires to Caliban -- found it riddled with Chaos and may have just left it to Dorn to crack open and take apart. The Lion had that kind of detachment from things. Or he my have charged in like he did and died.
In my honest opinion -- I see Dorn taking command of the Imperium and installing The Lion as the new "Warmaster". Therefore the Legions may have still been broken down to some extent, considering the Lion's past with the "Orders" of Caliban. Though its just as likely that Dorn's influence would have prevented this and the Legions would have remained.
Out of the remaining Primarchs -- Khan, Corax, Vulkan, Russ, Dorn, and Lion I think its more than likely the Imperium would have been divided into six different segmentums (similar to the ones of today with Ultima being smaller). The legions assigned to these areas would be the ones who defended them. Of coarse, over time, the Primarchs would die/vanish randomly for no apparent reason -- and then we'd just have big-ass chapters.
The chaos gods abandoned Horus most likely because they saw the can of whoop ass coming their way and wanted out of the way so as not to get fucked up!