Warhammer 40k would face the EXACT same problem as all other games... in fact possibly even worse. Since its story is ALL backstory... It has no REAL narrative or story just set pieces. A movie would have to invent a story OR bend over backwards trying to make the backstory into a viable story.
Depends on the author. Some 40k authors know how to actually write characters who have motivations and aren't automatons. Lots of other authors parade out faceless clone characters who say and do the rote things with the required # of combat scenes per book written in such and such a fashion....
Basically, 40k's fluff paired with a good screenwriter who knows how to craft interesting, small narratives could make it work. But I agree the sheer weight of backstory that has to be convincingly communicated to someone who knows nothing about 40k is a big problem. It's the kind of movie that would be 2 hours long and the first 30 minutes would be exposition.
The reason I think people shy away from a full-scale movie production of 40k is....the themes.
-Incredible amounts of Jesus allegories.
-One mustache away from a full blown Nazi reference.
-Extolling the virtues of:
- Fascism.
- Racial purity.
- Intolerance.
- Xenophobia.
- Religious fanaticism.
- Extreme violence.
- Genocide.
- Hatred.
- Self-mutilation.
-Daemon worship.
-Copious amounts of human sacrifice.
-Debasement.
-Humans enslaving humans as a matter of course.
-No compassion.
-No happy endings.
-No lessons to teach other than "Defy" and "Survive."
-No romantic side plot.
-Grim march of eternal grimness wot is grim.
-Hyper, almost cartoonish masculinity.
-Old timey dialog.
I mean, the list goes on. A 40k movie for a serious producer/studio is an enormous gamble. The potential for bad PR is huge. We've gone from the days of groups of parents freaking out about satanism to a hyper yet superficially aware populace that is hyper-opinionated about appearances, gender equality, morality.....
It's a potential landmine because it's not a sure thing it will take with general audiences, and it's an IP that doesn't rely on the same things as, say, comic books.
While I dearly love 40k, I think it's easy to think as a fan that it could be for a lot of lovers of sci-fi and fantasy. Truth is, it's a pretty nasty piece of work by modern standards.
Still, I think if someone did 40k subtly enough, increased of charging face first into the camera with it, it could draw people in. It's not possible to fit all of 40k into one movie, at all. And so if you ease audiences into it it might work.
As a fan though, my biggest reservation is the cheese factor the IP brings. It reads a little cheesy sometimes and, like, the Ultramarines movie showed me that you can't simply translate 40k lines into speech and have it sound right. For video games it's fine to be over the top but a full length movie has to be believable. I'm playing Battlefleet Gothic Armada right now and while the voice acting is quality.....I picture it in a movie and it comes across as hammy as fuck.
Like....imagine you're watching this dank, serious rendition of 40k in a movie when....the Orks burst on the scene and start jabbering in cockney for comic relief. We forgive a lot of that as fans, as gamers because it's just part of the IP. Dunno if that works for a movie. I know Orks can be done dank as fuck. It'd just take a good writer and director to see that and pick the right tone overall for the movie.
and a shit one at that
Shit because of the format and mechanics I'd like to add, not necessarily the story.