I'd make a cool morally grey movie focusing on rogues, somewhat along the lines of Fafhrd & the Grey Mouser series. Heck, forget D&D and just make a Lankhmar Movie.
Or do some kind of wild, over the top, Multiverse/Michael Moorcock movie. Jerry Cornelius or Elric or something.
Here's a long ramble on how I'd make the movie:
CHARACTERS:
Introduce a motley crew of characters that are thrown together by a common threat. The threat should be real, and scary, and compellingly "realistic". Not a chortling, scenery chewing villain like Jeremy Irons in D&D, but some kind of great evil akin to the Nazis or something. The characters should all be well developed within the time constraints of the first act, should follow mythic archetypes with flaws and have room to grow as the story progresses. Their individual quests for self discovery (and the suspense created by never knowing if they'll achieve them) should be introduced and play out during the overall quest to defeat the evil.
This interplay of overall and individual quests should create a lot of drama, and make the story more compelling, in much the same way as the motley group of characters from the Star Wars movies were popular (The rogue smuggler, his burly reliable wookie partner, the trickster r2-d2, the bumbling good natured C-3PO, the noble impulsive princess, the wise jedi guide and the stripling farmboy taking his first steps into a larger world {a character we all are at some time in our lives}). Heck, this same kind of dramatic interplay makes MTV's the Real World work as well (Will so & so overcome their fear of relationships? Will the house homophobic jerk learn how to be considerate? Will they all achieve their quest of joining the WWF? Are they Tough Enough? etc.).
They should all have their own motives, some of which are at odds with one another. They shouldn't get along and the audience won't know if the characters' interpersonal conflicts won't compromise the quest in the end.
THE ACTION SCENES:
Next, I'd follow the Spielberg-Lucas Formula of having 3 acts each with 3 mini acts. This means the story can take place in 3 interesting environments. The main characters should be introduced in an opening vignette which also introduces a villain or some connection to the main plot.
This is basically how Raiders of the Lost Ark works:
Act I - introduce everything by showing it in cool action scenes, *not telling*
a)vignette in the jungle, introduces the villain Belloq, and we learn what Indy is like by how he responds to danger in the temple - he's resourceful but picks bad friends...
b)introduce the quest at the college, we meet more characters and find out that Indy might we tackling something supernatural...the goal is the ark and that the Nazi's are the villains
c)climax: meet marion and cool action scene in the barfight in Nepal. The key to discovering the ark's location acquired
Act II - The quest for the Ark in Egypt
a) intrigue in Cairo
b) uncovering the ark
c) First major climax: 3 cool sequences back to back: escaping the well of souls, the fight at the plane, attacking the convoy (whew!)
Act III - The ark's mystery revealed
a) the nazi's steal it and marion
b) Indy chases them down, but gets caught...how will he ever get out of this one..?
c) the ark is opened...the mystery revealed...brief, satisfying conclusion.
STAR WARS also does this:
I - set on tatooine with aliens n' stuff
II - set in an impenetrable enemy fortress w/gun battles n' stuff
III - a dramatic space battle against impossible odds (you can't have a sci fi movie w/out space battles, after all)
I'd make a D&D movie following this same basic structure. I'd also include 3 staples of D&D to make it a true D&D movie without trying to awkwardly copy the game (like lame videogame movies do): The adventuring party, The Dungeon and treasure
Act I - introduce characters, villain all through action sequences in which you *show* who the characters are and what the fantasy world is like without *telling* (having characters make boring speeches about prophecies and stuff or describing how they're badasses instead of seeing it onscreen). The characters all meet and find they have a greater need to work together than be at odds (or do they?)
This scene should be set in a fantasy city like Lankhmar in which lots of cool stuff exists in a concentrated area, and there's a good reason for a group of adventurers to meet and form a party.
Act II - Journey through several picturesque environments culminating in a major extended set of action sequences, preferably in a killer dungeon. You need a good Dungeon in a Dungeons & Dragons movie, doncha? What makes dungeons appealing? Entering a dark, dangerous place where danger lurks at every turn--the goal is to capture the essence of what made D&D fun the first time you play it with lots of cool stuff to satisfy the game fans -- the ultimate dungeon crawl experience w/out a lot of D&D cliches. Enough stuff to keep the whole audience guessing.
The party faces a series of increasingly dangerous obstacles until they face a major battle that brings them together as a group, but casts doubt upon their ability to achieve their quest. The acquire magical treasure to help them in their quest.
Act III - The final battle
The party enters another even more dangerous environment that's different than the ones in the previous acts (another dimension, the abyss, the enemy fortress). Will party conflict prevent their victory? The bad guys should get the party in a bind, and through resourcefulness that's true to their characters, the party should win through. Unless there's going to be a sequel...