I'm kinda curious as to whether or not folks would accept a spin-off universe in which GW says: "Okay, this is What If Land in which has no connection to the current 40k canon-lore. Consider it us trying some new stuff. Don't like it? No problem. Stay playing the stuff you've grown to love, we'll keep supporting it just as we always have. But we'll do this new stuff, just to offer something else." In that, they officially move the 40k story forward in however manner they deem appropriate.
They did this with Battletech, and... it died. Pretty much the entire fanbase retreated from the Dark Age stuff back to Classical. At least that was my take on it.