Wild, reckless, terrifying speculation based on Sisters releasing, a Black Templar fleet in the Psychic Awakening video and the guesses that this means Endless Spells in space:
The Black Templars and Sisters worship the Emperor as a god - this comes to be true, and the awakening of psychic humanity gives rise to the summoning and of Loyalist/Imperial Daemons, so the Sisters summon Saints and Geminae Superia, and Templars summon Legion of the Damned as manifestations of the Emperor’s will. If the end of Master of Mankind is anything to go by, down the line we get Legion of the Damned Ferrus Mannus as an Imperial Daemon Primarch.
/tinfoil hat
In that case I for one am looking forward to Ultra Instinct Straken and Sly Marbo for Imperial Guard.