Right on TL. I just finished the single player campaign (again) on insane with the new patch...which gives Chaos quite an upgrade hehe.
(Spoilers from original campaign!)
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Seems like they could have gone in order, 5 campaigns:
Eldar trapping the physical embodiment of Khorne
Orks fighting Imperial Guard and slaughtering civilians on Tartarus to incite Space Marine incursion
Space Marine single player campaign in original (without the newbie missions lol)
Chaos, with their released blood god striking back wherever they choose in an orgy of death.
Lastly, since the expansion is Imperial Guard, Imperial Guard and Space Marines assaulting and containing the Khorne-Chaos threat to the ultimate no-real-change ending lol.
Thus is WH40k...billions die for no net gain on any front, but a lot of fun doing it
Now, if Relic wasn't relatively small and slow to release things they could add a campaign where the Inquisitors (and Adeptus Sororitas or Grey Knights), who were clearly up to something before the Space Marines got there, could fight Chaos and Orky forces. They could also fit 'Nids in there, because there's lots of blood there for the blood god, right? They could probably even fit Dark Eldar in the early stages of the campaign as Chaos allies for Eldar slaves or something. Of course, I don't know 'when' the eldar did all of that - might have been a Warhammer fantasy thing for all I know.