Board Thread:Warhammer 40k General Discussion/@comment-24821739-20140604204305/@comment-24944355-20140611010317

Attank it's true demons can enter the world but only for a short time. They can exist permanently only in the warp or places where the warp bleeds into reality like the eye of terror. Also the chaos gods themselves can't leave the warp, at least not without sacrificing most of there power which they would never do even if it was possible. The chaos gods are simply to powerful and to bound to the warp to exist in reality and The Emperor now falls into the same category, only the golden throne enables him to retain a thread to the real world now.

Also there are several Primarchs who could return, the lion and Guilliman being the most likely. No one has any idea where Corax is, Russ is probably imprisoned by Magnus, Dorn and Jaghatai are probably dead and Vulkan is probably one of the treasures the Salamanders are searching for (as hinted at the end of Unremembered Empire).