Fuck the tired, straight up fantasy angle. The only way for this to be a critical and commercial success is with genuine subterfuge on the part of the actual D&D world.
I envision it as a group of D&D nerds, back in the 70s when it was a brand new thing, fighting monsters in the family rec room. The style would change to CG for the fight scenes. But the dudes fighting would be be these pimply, bespectacled teenagers who are completely kicking ass, etc. in their game. All the school girls they have crushes on would be maidens to save in their game.
It would be perfect for Wes Anderson. I think it would be hilarious and awesome.