I had always thought there were two ways to do the D&D movie
1) Do the standard High Fantasy Road Trip story just on Faerûn. 3-5 heroes of various skill sets (Caster, Fighter, and Thief OR Divine Caster Arcane Caster, Ranged Fighter, Melee Fighter and Thief) trek across the Forgotten Realms to battle a great evil and passing through famous locations (Waterdeep, Baulder's Gate) and fighting iconic Monsters pulled straight from the Manual (Beholders, Flayers, at least one Dragon probably Red to call back to the Basic Set cover art).
2) The above, but with kids/teens from Earth being pulled to Faerûn through some plot coupon. (the Amulet of the Planes would be the obvious one) They could have been D&D players beforehand or just random folks or a mix (have the token nerd(s) realize where they are and use his knowledge of the setting to their advantage. Get the wish fulfilment in there) but have them travel to fight the Big Bad (who just so happens to have a way back to Earth). Maybe some (or all) of them stay, feeling more at home in the Relmes then back on Earth. Make it a metaphor for growing up or something.
This shouldn't be too impossible to pull off. Right?