Board Thread:Warhammer 40k General Discussion/@comment-8881468-20150204091114/@comment-25618639-20150212191531

This is the wiki's version of how he would "return"

The concept of the Star Child in relation to the Emperor is similar to that of the Eldar's potential God Ynnead, the Lord of the Dead, who according to their belief will form in the Infinity Circuits of the Eldar Craftworlds when the last of their race has died on the physical plane. The belief in the Star Child is currently considered by the Inquisition to be a vile heresy spread by Chaos Cultists ofTzeentch, though the truth of the matter might be otherwise. The concept has two aspects: the first is that the Emperor's soul exists in the Warp where it will form a new entity upon the Emperor's death -- the Star Child -- who will become the Emperor Reborn, and the second is that the Emperor during his long life on Terra has many living descendants. Over the almost 50,000 years the Emperor walked hidden among Mankind, he formed many families and fathered many children. His male descendants have inherited direct portions of the Emperor's genome and they are immortal like their father, though unlike him they are sterile in every case. His female descendants are simply normal human women, though they may exhibit potent psychic abilities, particularly forms of precognition.

Unlike the Emperor, who is the greatest psyker ever born in the galaxy, the Emperor's scions are actually psychic "blanks" or "nulls" possessing no presence in the Warp. As a result, they are both undetectable by psychic means and cannot be affected in any way by psychic or sorcerous powers. A hidden group within the Imperium that call themselves the Illuminati know of the existence of the Emperor's scions, and are also aware that the Emperor is failing as his life slowly gutters out despite the creaking ministrations of the Golden Throne. They also know of the Fall of the Eldarwhich gave birth to the Chaos God Slaanesh and seek to prevent Mankind's own fall to Chaos. The fall of Man to the Ruinous Powers would create a new, fifth major Chaos God in the Immaterium, and with its birth, the Warp would once again intrude into realspace for a number of millennia, encompassing the entire galaxy in massive Warp Storms, as during the Age of Strife. The Illuminati, who as a group are those incredibly rare individuals who managed to actually survive and defeat a daemonic possession of their bodies through sheer willpower, seek out the Emperor's descendants and tell them of their true heritage. With the realisation that this knowledge brings, the biological sons of the Emperor then become known as the "Sensei." The Illuminati gather the Sensei together, protect them from the Inquisition, and pave the way for the rebirth of the Emperor. Their plan is ultimately to sacrifice the Sensei to the Emperor, possibly at the moment the Emperor's body finally fails upon the Golden Throne; he will be renewed by the Sensei's life forces, and will be reborn and regenerated as the Sensei-Emperor to again lead his race in person, stepping renewed from the Golden Throne. While the Star Child background was introduced in the original version of Warhammer 40,000, and dismissed as heretical in the 3rd Edition in favor of the more vague "Iron Men and Stone Men" history of an ancient cybernetic conflict between humanity and a race of artificial intelligences they created during the Dark Age of Technology, the Thorian faction of the Inquisition believes in a similar concept concerning the Emperor's eventual rebirth and reincarnation.

The Emperor is the incarnation of the extinct shamans of ancient Earth who, with their prophetic powers and connection to the Warp in its natural and uncorrupted form, had guided the various peoples of ancient Man. According to the Illuminati's beliefs, after Horus rebelled and mortally wounded the Emperor's physical body during their final battle aboard his Battle Barge Vengeful Spirit, the Emperor's body and soul could no longer remain as one; his soul melted into the Immaterium and only a tiny core of the Emperor's humanity remained whole. This spark of the Emperor's soul was like a small child in a tiny reed boat adrift in the chaotic eddies of the Warp. Since the Emperor's soul survived, there was a possibility that his whole essence could be reborn into the physical world once more in a new physical body. In the same way the ancient shamans died together to reincarnate as the single man who would become the Emperor, the Emperor's death could herald the birth of a new savior for humanity. That time would lie far in the future, when Mankind's collective desire for a new savior would strengthen the core of the Emperor's soul in the Warp and rekindle it to new life. The soul of the Emperor adrift in the Warp is the being referred to by the Illuminati as the Star Child. The humans that were left in charge of the Imperium after the Horus Heresy and the loss of all the Primarchs had no real understanding of what had happened to the Emperor. Though the Emperor's body continued to live within the Golden Throne and his potent mind continued to be a beacon for humanity in the form of the Astronomican, his soul is a new, benevolent God of the Warp, the God of Humanity, waiting to be born. The Star Child is also believed by some Illuminati to be the pure compassion of his soul which the Emperor thrust from himself into the Warp in order to possess the determination required to eradicate the soul of his most beloved Primarch and son, the Warmaster Horus.

Taken directly from the Emperor of man article, it says he would be reborn into a new body.