IMO if there ever was a 40k movie, they couldn't touch the Heresy, primarchs, or any of that shit, that's faaar too much info to shove onto an audience in one movie. The movie would have to be a small story, perhaps a "first time ever" story, like meeting the tau or fighting the nids for the first time, so the audience can feel a connection with the characters, as they too are in a new situation.
Perhaps after 3-4 movies in universe would they be able to breach a story requiring as much fluff knowledge as the HH.