I realize this is a somewhat contentious topic; I'm not judging anyone for liking WH40K- while I am no lore expert I do enjoy the setting, but I believe it is important to at least acknowledge or confront this, in much the same way that one must as far as I'm concerned confront the xenophobia of Lovecraft and how that is largely inextricably linked to his works, no matter if you still appreciate the work in itself in the end.
Anyway, right now we're just taking that as a given and exploring the worldbuilding implications. If you disagree with the premise that's fine, but just roll with it so we can have fun worldbuilding.
At risk of alienating even more people, I asked on my server if anyone could generate art of Warhammer 40K meets Hieronymus Bosch and Jack Kirby, and Peakrill obliged with some wonderful pieces, to help bring this to life. Do I think these are as good as, or as close to the ideal of what I'm looking for, as what a human artist could make? Probably not. But I was never going to commission an artist to make these anyway.
If The Imperium is almost like a dark ages post apocalyptic take on the post Roman empire world, this would be more like a post post apocalyptic take, something closer to the more modern understanding of history or at least my understanding, that while in some ways it was a period of loss of knowledge and infrastructure, in other ways they were more progressive and functioning reasonably well just on a smaller scale. In this world, it would be like Star Trek almost, like magical philosopher explorers, encountering all the craziness out in the universe with technology they only barely understand, wonders and reminders of what they can one day become.
The space orks are like rowdy punks in some cases, or psychedelic social anarchists, the Eldar are a little uptight but they'll take you out for a good meal and even if it's just to show off just take the lunch ya know? The tyrranid are an invasive species that need to be dealt with by social environmental institutions, like Space Rangers of galactic parks and recreation.
Oh and Chaos are just space Nazis of course.
Rather than looking like a mix of grimey industry and dark fantasy, it would be whimsical and Weird and Wonderful, like Hieronymos Bosch. Eh and throw Jack Kirby in there too. Decidedly not Moebius I think that would actually be too obvious and take away from it.
In the same way that I want for instance the tyrranid to still be a threat, but the way they confront that threat is profoundly different, and I think dealing with fascism and hate and violence is still something they should have to confront and I like the play on "Law and Order" types actually just creating Chaos, that the dichotomy of Order and Chaos is often just a fabrication or means of weaponization.