I'd make it a soft sequel to the '80s D&D cartoon.
No. Really.
I'd have a group of 20 somethings pulled from our world into "the world of Dungeons and Dragons" where they find four magic items. Likely a sword, wand, holy symbol, and dagger to fit the four iconic character roles (and not overlap with the existing items). This works because magic items allow the "heroes" to be badasses despite having no skill. So a sword that cuts anything, a wand that cast spells, a holy symbol that heals, and a dagger that returns when thrown even the playing field between the everymen protagonists and all the monsters they fight.
The characters should likely find some local (an elf maybe?) to explain the world and plot as they stumble into adventure, while trying to find their way home. Throw in a nice mix of D&D monsters and classic beasts, a simple dungeon crawl, and generic quest. Volia.
The advantage of this would be the "metaphor" for the game, with regular people entering a fantasy world where they can be heroes. It also sidesteps the generic nature of the game worlds, which are advantageous for the game (hitting all the tropes) but would make a movie seem, well, cliche. It provides a hook to separate the D&D movie from all the more generic fantasy movies, which have the advantage of being based on a book or other property with a firm story and known characters.
Plus, it would enable the protagonists to make pop culture references and crack wise despite the serious trouble they're in. Which feels a lot like how the game plays.
It'd almost be advantageous to have the plot be generic. "Gather ye the seven parts of the rod of wonder. Then ye may use it to open the portal to get home. Keep them away from the forces of darkness." Just so the protagonists can lampshade the plot. "But, y'know, what if we throw the parts in the ocean? Or in a really, really deep hole?" And, of course, one of the people sending the heroes on the quest turns out to be the villain and betrays the party. Which, of course, the heroes knew and anticipated. "Oh come on: you're bald, dressed in all black, and have a goatee. You might as well have cackled maniacally."
I'd also slip in a reference to the original cartoon series, suggesting the fate of the old heroes. Likely with the new heroes' mentor revealing himself to be from the real world, but having chosen to stay behind and be a hero. Which, of course, the new group would do as well, opting to remain and be heroes and get home "someday". Because sequels.
And since we're fanwanking already, lets throw in a "lost" episode of the cartoon as a bonus feature on the DVD. Although, I imagine the rights to that are a hellish muddle.