I read somewhere something about how all the psychers that are sacrificed to the emperor to keep the astronomican running actualy join with his soul in the warp. And since the warp is spawned and controlled with belief and feelings, plus-quadrillion humans believing the emperor to be a real god would probably actualy cause the emperor to become a real god.
This, among other things, drives the theories behind the idea that the only thing holding the emperor back is his physical shell that holds the core of his soul, so when he actualy dies, his soul would unite with the other fragments, charge up on all the belief of everyone in the empire + the souls of everyone that got sacrificed to keep the astronomican/the golden throne/the emperor running and become an actual god capable of simultaneously punching every chaos god in their metaphysical faces.
There is a real reason as to why chaos calls the emperor "the anathema", and its not just because he was the most powerful warlord in the galaxy before, its because his very existence is in direct opposition to chaos, which is why the emperor cannot be corrupted by chaos, and why chaos considers him an actual enemy rather then just another being to use as a pawn.
In fact, the big E might be chaos' only true enemy. Every other being that the chaos gods cannot control do not seem to be in direct opposition to it. The only other exception would MAYBE be the Eldar's super neon genesis evangelion batman gambit, since they're apparently chanelling their souls into craftworlds to create a new, artificial god, that is supposed to free them from slaanesh permanently or somesuch. Another real threat would possibly be Gork and Mork, if the orks actualy manage to join up under a single warboss to channel their insane crowd mentality powers to create them.