Over the years, there are a number of things in the fluff that kinda annoy or grate me. These can be very petty and idiosyncratic, of course, but I've sometimes invoked some headcanon, or fantasized about how it would be with a different angle. So, in the interest of just shooting the amphetamine parrot, here is the thread where you can tell us all about the things you would have seen done differently.
For the purposes of this thread, assume that you're at the writer's table at its very inception, not being let into Games Workshop in the modern day.

---------
So here are some of mine:
- I've never really been a fan of the Grey Knights. I just find their background a bit naff. They're so often sold as future space knights, but that's like... ALL space marines. Additionally, they are said to be superior to ordinary marines, which in my opinion is a niche already occupied by Custodes, so there's too much overlapping, imho. Then there's a vague overlapping with the Death Watch too, in my opinion, with the coordination with the Inquisition. I don't know, I'm just not a fan. In my opinion, I'd prefer if the Grey Knights were instead a specialized arm of Deathwatch comprised of librarians and what have you, not a specific Chapter that's specialer than the special. Custodes get to be uber-ubermarines.
- Necrontyr: Okay, first off, that name is dumb and needs to go. Call them literally anything that's not just a pun, please. Secondly, part of their lore was always that they resented the Old Ones because their lives were so short. Pretty every retelling of this lore makes it pretty clear, even if read between the lines, that their lifespan was short by HUMAN standards. This just ruins my immersion, personally, since it shows that this is all some the Udders of Thoth written by a real-life human author seeing things from a real-life human perspective. The Necrontyr lived at a time when humans weren't even evolved in the 40k universe, so how would this comparison be relevant or interesting? I think the simplest change would simply be to say that the Necrontyr lived short lives *in comparison to the night-immortal Old Ones*. Their resentment was due to the perceived injustice when seeing themselves in relation to their neighbors. That fixes it pretty neatly, honestly.
----
Got any hot takes of your own?