What timeline would be necessary to make Warhammer 40k a mainstream popular setting . While at the same time keeping lore similar to what it's is now and keeping it's grim dark theme's.
Doom is set in WH40K universe and Blizzard gets Games Workshop license for Warcraft and Starcraft. Thus some of the biggest games franchises gave a big popularity boost to especially WH40K and fill GW coffers to the point where they make a video series to take advantage of huge popularity in South Korea. The series is also a moderate success elswhere, emboldening GW to fuhrterly expand into television and perhaps even cinema market.
GW are not as draconian* with their protection of their IP and collaborate more with artists making content
There was a pretty good German fan made movie (Inquisitor?) that literally had to go underground and recently we had this rather good 'effort' which gives an indication of what could be done.
*I once visited Mantic Games old HQ in Nottingham and they used to 'frame' GW Cease and Desist letters and hang them in the Toilet