In examining the idea of creating a different vision of the Imperium of WH40K, the interesting thing is that most of the institutions of the Imperium have the genesis at it’s beginning and either remain relatively unchanged or were twisted by events of the Horus Heresy and the long period of survival since.
The Council of Terra evolved from the War Council of Great Crusade and eventually evolved further into Senatorum Imperialis (the High Lords of Terra). The Adeptus Astronomica, the Adeptus Astra Telepathica, the Adeptus Administratum, the Officio Assassinorum, the Warrants of Trade of the Rogue Traders and the Navis Nobilite all certainly date back to this time. The Imperial Army devolved into the Imperial Navy and the Imperial Guard, while the Thunder Warriors of the Wars of Unification were supplanted by the Grey Legions which then became the Adeptus Astartes Legions led by the Primarchs and were then divided into the Space Marine Chapters post-Rebellion.
The Adeptus Mechanicus certainly existed at this point but it is arguably going to have a somewhat different version of the Cult Mechanicus given the lack of devolution and the influence of the Imperial Creed upon the tenants of the Sixteen Universal Laws and the worship of the Omnimessiah.
The Imperial Truth and the role of the Iterators was jettisoned for the Imperial Creed and Cult along with the rise of the Adeptus Ministorum. It is likely that some form of the Inquisition existed in this time period as well – certainly things like the Black Ships did, so some sort of mechanism similar to at least the Inquisition had to have existed – plus something which would have been an intelligence network (though these likely have their genesis in the immediate lead-up to the Rebellion). Things which most certainly didn’t exist are the Adeptus Ministorum as well as (somewhat obviously) the Adeptus Sororitas. Finally, the role of the Legio Custodes was significantly different in that era.
There are also a few things that would have to be resolved – is this an alternate timeline? Or is it, and this is the idea which I have, is there just a transplanted set of elements into some other part of the galaxy that have survived and prospered without the baggage of the Horus Heresy?
If that is the case there are a series of questions or issues that would need to be resolved or addressed to maintain the general “sense of place”.
- The Astronominicon
- Space Marines
- The Imperial Cult
- The Ruinous Powers
- The Xenos
- The Ritual of Soul-Binding
Interestingly, there are a number of elements that would perhaps help resolve some of these issues. There is original plan for the Golden Throne and the Eldar Webway, the two lost Space Marine Legions from the First Founding, the Expeditionary Fleets of the Great Crusade, the Sensei, the Old Ones and the truth behind the Necrons and the C’tan, plus the existence of things like the Jericho Maw Warp Gate that make such an alternate setting possible, not to mention the existence of the Jokaero. Even the existence of not just the Eldar Craft Worlds, but the Exodites and the Dark Eldar (and the Harlequins) point towards the possibility of another version of the Eldar surviving the Fall.
All of this makes for a relatively intriguing potential collection of elements and mysteries for the proto-setting.