As much as Hollywood wouldn't do it... I think the best way to establish the D&D "film brand" is to start small. You need to establish the fantasy world the characters live in, you need to establish many of the tropes of "D&D" that live in this world, you need to establish the premise of "adventuring", which is what D&D is about (and thereby differentiating it from all the other fantasy films / tv shows out there.)
What you need to do is ape the plot of the fantasy films that do all that in spades.
You need to ape The Goonies mixed with some Raiders of the Lost Ark.
A band of adventurers finds a treasure map, and races against another group of "evil" adventurers to reach a huge treasure in order to save their town. They face off against many other enemies and obstacles along the way... and the climax of the film is a trek through a booby-trapped dungeon.
You don't get more Dungeons & Dragons than that.
Leave the political machinations, the massive monster invasions, the Chosen One vs The World Threatening Event for a couple films down the line. We've GOTTEN those in the Lord of the Rings saga, The Hobbit, soon to be Warcraft, Game of Thrones. Dungeons & Dragons is smaller than that. At least, GOOD Dungeons & Dragons is smaller than that. It's about adventurers of different stripes coming together to find some treasure and/or rescue someone of import. Start there. Establish the premise of the game in the movie. Use the opening scene of Raiders to establish your adventurer band "in media res" invading a small dungeon... then once they acquire their treasure, transition into an scoped outline of The Goonies story for the rest. Their small town in trouble (maybe kidnappings from the outlying farms.) The Adventurers discover the humanoid bands that are taking their victims on behalf of another evil group... they learn of a McGuffin the group is trying to get via a map or something... they get hired to go get it first and they then fight their way through some old ruins... make their way into the dungeons and face off against the monsters, members of the other group and the deadly traps therein... and by the end acquire it.
Small, simple, and you give yourself the time to establish the tropes of the game that make this fantasy a bit different than the other fantasy films and worlds out there.