I think they could do it, just about, but we’d definitely see a pretty huge discontinuity between “what the fans want” and “what a general audience wants” (even if it’s an SF-oriented audience).
“40K fans” (self-described, typically) basically seem to want only one thing and it’s disgusting. That is, SPHESS MARINE boots stomping on the faces of the Emperor’s enemies, forever. Like that’s the whole of what they want - Space Marine Violence-Porn. Endless episodes of space marines busting in and murdering “filthy xenos” or “dirty heretics”. Maybe occasionally dying heroically after reciting some battle-chant. But mostly just slaughtering. Oh and they really aren’t interested in any enemy but Chaos. Orks, Eldar, Tyranids (except Genestealers), Tau, and so on? They can all go home for the “40K fans”.
And that’s just never going to happen, because like only a few million people would even initially watch it, and by the end of the first series they’d be down to a few hundred thousand.
There is supposedly an Inquisitor Eisenhorn show being made by Frank Spotnitz, who was involved with the X-Files and created Millennium and produced a whole bunch of TV shows who could best be described as “Oh god so dull…” (particularly The Man in the High Castle). That could work, but I think they’re going to need to make major changes, and turn down the “grizzled” factor of most of the characters involved to get any size of audience that matters.
And I dread to think the FX budget. Altered Carbon cost like $150m, and it was clear that was so ruinous the second season is desperately saving money in some very questionable ways. And that had a much broader basic audience appeal.
Even if they follow the Mandalorian model, they ain’t got no Baby Yoda, that ain’t the sort of thing that happens in the 40K universe. And they don’t have the broad audience recognition and cross-gender appeal SW has. And if they do by some miracle make a show that’s actually good and watchable, all the fans will be moaning endlessly and just crying for MOAR SPHESS MARINES, or because there will inevitably be characters who aren’t 9’ male white ubermensch in it, will no doubt be muttering about “The SJWs” and so on too. I mean I guess that’s fine if the show works, but it’s going to be very annoying.
Fantasy has a potentially far more diverse and interesting cast, and a more fun and less alienating setting, but it seems like the set/FX budget there would be potentially even higher. As a setting totally based on Moorcock and Tolkien (and people see Moorcockian influence as “D&D”, sadly), it seems like it might have difficulty standing out from the non-tiny number of other fantasy shows in development.