NOTE - These are the combined version of two of my posts on SB being crossposted here.
I've been thinking of writing a world where the Emperor decided to not create his own empire but manipulate the Mechanicum or something (referred to from now on as Mech), basically a universe where the Imperium never existed. The Mech would expand after the warp storms ended, though not Great Crusade-style or scale, they do have colonial interests evidenced by the fact that the Mech tried to expand during the Age of Strife by sending colony ships during lulls.
I also thought of an interesting societal/population issue that would drive this expansion. The thing is that Forge-Fanes and many in the hierarchy of the Mech are practically immortal so unless one dies which is rare, Forge-Fanes from the 26th Millenium are the same ones as in the 28th Millenium. This causes the promotion of new tech-priests to be practically impossible as a lot of the old guard is still there. This means that there are new tech priests being churned out with nowhere to go. I've also thought of what if they just stopped training new priests, this goes into the contract between the menials and the priests, that being one of the clauses is that they train smart menials into tech-priests. If they stopped training new priests, many tech-priests and menials would rise up in rebellion.
This issue is mostly being solved by conquering the solar system but as ground-space runs out, this is becoming more and more of an issue, by the 30th Millenium, there were a lot of tech-priests with not much to do causing a flourishing of philosophy and different political ideologies and it was almost coming to blows and internal warfare. However, the disappearance of warp-storms caused by Slaanesh's birth scream suddenly solved this issue by regular interstellar travel being possible again. Thus, the great diaspora of tech-priests began, with all sorts of differing tech-priest groups going and claiming planets for themselves. Some groups feigned a disappearance and split from the Mech as a whole.
"Mechanicum" worlds where they practiced reckless innovation, others where the priests attempted to merge with the warp (warp cyberization cults), worlds with civil and human rights for everybody, democracies, autocracies, theocracies, basically anything and everything you can think of. Of course, more traditional Mech worlds were the most numerous and they obeyed Mars in all things.
The Mech would also mostly take on a policy of diplomatic conquering taking worlds as vassals in exchange for tech from the Mech. Then slowly integrating them more and more into the Mechanicum until they're Mech worlds.
These are most of the coherent thoughts I've had about this, I've came up with this idea a couple of weeks ago and I've been very busy. Some less coherent ideas are the devastation and fracturing of the Mechanicum after they win a pyrrhic victory against the Ullanor Orks. An Ultramarian league in sort same vein as the
second Athenian league, with Macragge having a lot of influence over the other planets and them uniting being partially for protection from the Mechanicum colonization and vassalization.
The Primarchs might never even come into existence, instead, Gene-enhanced diplomatic corps might be what he goes for. The military conquest of highly developed industrial worlds just seems extremely inefficient as they're going to want to rebel and the siege will be a meatgrinder. Combine that with the fact that many of these Technocratic states originated not too long ago from Mars, and most have a lot of cultural similarities making it easier for them to integrate with each other.
Imagine that, diplomats that can read a tech-priest's emotions and desires by the slightest twitch of a mechandrite, interpret binary without cybernetic augmentation, and have a lot of charisma, oratory skill, and a good understanding of the mindset of Tech-priests.. Using these diplomatic corps, the Emperor would probably begin to consolidate the Mechanicum Tech states, he would also use the external threat of the Ullanor Orks as a reason to least join together temporarily to oppose the Orks.
EDIT - I don't think the Emperor would make himself really known, instead using Agents like he did for most of his lifetime and if he did do something significant, he would do while disguised as a menial or techpriest or someone else then quietly disappear afterward. I think using agents and being subtle is his usual MO and that making the Imperium with him at his head is a deviation because of extraordinary circumstances, in my timeline, he continued to be subtle as per his usual strategy.
By the way, thank you for the feedback and add ons.
I want some criticism and feedback from y'all about this along with helping me flesh out this world I am building.