I unfortunately know little about Warhammer 40,0000 so I couldn't give anyone a proper fanfiction about if I tried, it's also so big and complicated with so many different factions and subfactions, worlds and species, that I couldn't keep track of them all if I tried unlike Star Wars (watched all six movies by ten) I wasn't raised with Warhammer 40,000.
So, this is a prompt with some guidelines for anyone who wishes to give it a go.
Title: Mother of the Primarchs or something similar.
The Primarchs according to the wiki page aren't traditional clones but is similar enough to the process, it also says that the Primarchs DNA wasn't purely from the emperor there were others mixed into it as well.
I was thinking that the emperor needed to add human female DNA along with everything else, what better DNA as the "mother" than the Godslayer Ranma Saotome?
Ranma and the Emperor fight, Ranma gives the emperor a decent challenge (have no idea what the emperor's combat abilities are capable of so the fight could be closer) before being defeated, I think the reason for the fight being Ranma disagreeing with how the emperor desires to control humanity and destroying its culture, heritage and history or something like that. The emperor impressed by Ranma' abilities (as Ranma isn't a pysker, doesn't have a pact with some god, nor technological enhancements, nor is whatever the emperor is, rather Ranam is an ordinary human who managed to break the limit) rather than kill Ranma, the emperor keeps him/her (you can decide whether Ranma is locked or not and if locked reasons for it) in suspended animation and later used the female forms DNA for the Primarchs (I suppose that makes her grandmother to the space marines).
Ranma is suspeneded in a secret laboratory in the Inner palace, somehow the guards discovered the lab and investigated it. They discover the still living Ranma inside, from the data they read about Ranma's exploits and powers and she is the "mother" of the primarchs. Guillman is called and makes the decision to free Ranma. Ranma's reaction to everything from the state of mankind to having children to the state of the emperor can be up to you. But ultimately Ranma decides to help the Imperium to save mankind (maybe add if you want trying to find all her children and free the ones that are corrupted, spiritual energy could be useful as an explanation for that and for purging chaos corrupted worlds).
What happens from here is up to, as long as Ranma isn't corrupted, good luck.