I think the most likely reason a D&D movie will fail is the same reason the first D&D movie failed: Trying to do too much.
Where did it go wrong?
(1a) "This is a D&D movie right? So let's have dragons and dungeons and dungeons and lotsa dragons when we save the princess (queen) at the end!" Way too much stuff going on.
(1b) Letting the name "Dungeons & Dragons" get in the way of a good tale is a terrible idea.
(2) Too many characters. The movie did not let any of them (except me, Snails) breathe.
(3) Not understanding the genre properly. Yes, a heroic fantasy will always have a good helping of melodrama, but melodrama does not need to be dry. Star Wars, Babylon 5, Pirates of the Carribean all blended in humor without being goofy. Well drawn melodrama characters have a lot of room for humor, because the characters, by their fundamental nature, have blind spots that lead to interesting misunderstandings (e.g. Season 7 of Deep Space 9). In fact, melodrama without properly blended in humor is going to feel boring or childish to typical audience members.
(4) #2 + #3 = the actors all look wooden. There was nothing wrong with the cast, some of them had fine careers both before and after. Write material that competent actors will be able to work with, rather than hoping the next Harrison Ford or Sean Connery will rescue the movie from weak writing.
(5) An amazing world of fantastic fantasy stuff is not automatically a plus. It can burn through effects budget quickly without endearing the movie to the audience. IMO Willow was one of the better pre-LotR fantasy movies; the story made enough sense without overwhelming us with weird stuff. Likewise, the first Harry Potter story is incredibly small plotwise, because Rowlings did not want to overwhelm with her weird world -- she wanted us to get to know the main characters instead.
If D&D is intended to be a movie franchise, too much is more likely to be lethal than too little.