Speaking of lore, I never really like the Horus heresy getting novelized. I mean exploring DA TRU history behind the myth is cool and all but from what I have gleaned, the events of 30k are a mostly faithful retelling.
The Emperor and the Primarchs are in fact demigods, Sanguinius did in fact have literal angel wings and the society of the Great crusade Imperium is pretty much unchanged (everyone swinging the space-temple look on their ships and stuff) even though everyone speaks like a stuck up Reddit atheist, cuz muh irony lol lmao!!
I think instead it should've been something like have the whole thing having been much more mundane: the real Emperor was an ordinary transhuman warlord that was oddly benevolent (though ironically his comatose near-corpse now actually is a very powerful god, because the warp works on feelings and belief and it's got an entire interstellar empire that basically exists to empower it)
the real Primarchs were just gifted rulers and scholars and the like which he recruited as the GC went on and became his greatest generals, after the Heresy as history passed into myth the Emperor became deified and the Primarchs became deified with him and the scattering/Emperor's Sons story was invented to explain their supposed divine origin,
the Space Marine geneseeds were unspectacular conventional biotech; which the current Imperium has long ago lost a lot of the knowledge involved, but can still imperfectly replicate the procedure in a cargo-cult fashion, and has mythologized its origins. It fits with the deteriorist themes of the setting. Though thanks to the warp, the geneseeds have become far more mystical...
I'd also have GC-era Imperium have an aesthetic much more resembling Some generic mil-sci-fi setting. This could lead to some neat short story abt a rogue trader team stumbling upon some derelict ship from that time. The derelict would have an aesthetic like the Nostromo from Alien, functional and industrial and with a hint of the "space cathedral" look but as something that emerges naturally from the vast impersonal machinery of the ship, without the baroque quality that 40K-era Imperial ships have. They'd find things that look kind of like power swords, power fists, etc., but in the tool lockers. They'd access the ship's logs and personal diaries of the crew and notice some odd things, like the names of Primarchs rolling easily off the tongues of the former crew. Them being addressed not as the demigods they have become by the time of the Grim-darkness of the far future but as modern-day pencil pushers and politicians.
The conceit would be to imply that the early Imperium was a profoundly different sort of society from the 40K Imperium, in a way that 40K-era humans might not pick up on but real-world readers would.
