Making the mext DnD movie should be like making a 3.5 monster (where you start with the CR): know your budget and what you can do with it. The DnD movie had too low a budget to feature a fully-rendered magical empire with tons of spellcasters, a fully-rendered elven treetop village, a fully-rendered battle with hundreds of gold and red dragons and a beholder, to boot...
A DnD movie might follow a group of 6 mercenaries hired to root out a nest of evil (ruined temple or somesuch): a knight (fighter), a wizened sage (wizard), a pious templar (cleric), a smart-aleck thief (rogue), a dwarf and an elf. Cast the kid who played Peter Pan to be the elf and a short beared guy to be the dwarf and you avoid the need for altering people's sizes (a reason why a halfling would be a bad idea, unless you add a 10-year-old to the cast to play one).
Get the make-up artists from Buffy and Angel to reproduce hallmark DnD races, like orcs, bugbears and hobgoblins. Throw in a troll or two. And end up with hints of a larger conspiracy involving the return of a beast of legend: a fire-breathing red dragon. Oh, and add familiar names to the movie to please the fans, like having the evil temple follow Vecna or Tiamat, having the wizard cast magic missile (and call it that) and having the cleric turn undead using a Pelor symbol.