I suppose it depends on how they deal with the heresy issue, but without spoilers so far, I think two possible primarchs could return, Roboute could heal and come out fighting, the traitor primarchs would sally forth, and it would be Imperial Chaos face off.
I think Vulkan h'stann would be wiped out, but would find Vulkan. Calgar would die, and possibly Dorn severely wounded. I think the craftworlds may come out in support of the Imperium, but suffer critical damage. I think necrons would side with chaos and stab them in the back. The orks and 'nids would take advantage, and tau would side with imperium for the greater good.
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I suppose it depends on how they deal with the heresy issue, but without spoilers so far, I think two possible primarchs could return, Roboute could heal and come out fighting, the traitor primarchs would sally forth, and it would be Imperial Chaos face off.
I think Vulkan h'stann would be wiped out, but would find Vulkan. Calgar would die, and possibly Dorn severely wounded. I think the craftworlds may come out in support of the Imperium, but suffer critical damage. I think necrons would side with chaos and stab them in the back. The orks and 'nids would take advantage, and tau would side with imperium for the greater good.