To be a D&D movie you don't need to include every D&D thing as a cameo. As an example the beholders in the first movie were there for no aparent reason. Pick a module/story and translate it to film. Only include the monsters and items needed for that module/story. When playing D&D I do not expect to encounter everything iconic from D&D. My characters have never encountered Mind Flayers, Beholders, Displacer Beasts, Owlbears, or Ettins; nor have they aquired Vorpal swords, Staffs of the Magi, or any part of Vecna. So pick an atagonist and surround him with believable minions and save the rest for the next movie. Which was better, the first batman movie where he dealt only with the Joker, or the last one where everyone good and bad was a part of the movie.