If a mainstream Warhammer 40,000 series were to be any success, I think it would be by fighting upstream of many of the current trends of high-budget IP-driven television, specifically by having a highly episodic structure rather than a highly serial one.

Imagine it, if you like, as a suitably grimdark version of classic Star Trek. Make the heroes (heroes in the narrative structure sense, not the moral sense; this is still 40k) people with a reason to trot around the galaxy, be they an Inquisitorial cell, an Imperial Guard regiment, a Rogue Trader fleet, Arbites enforcers, Assassins, or an Administratum audit team. Have every episode be a relatively self-contained story with its own central threat. Imply the vast scale and variety of the Imperium with a different world every episode. Minimize references to greater-scope 40k lore like the Horus Heresy or the Black Crusades, keep the focus on our main cast and their individual adventures. Most importantly, let every major army get an episode with it in focus, as either allies or enemies.

I expect basically none of this advice to be followed; highly-serialized, fanservice-packed melodrama mostly about Space Marines, here we go.