I like a lot of the suggestions we've seen so far...I shudder at others.
Like it or not, I think it a foregone conclusion that they will insist the script be set in Forgotten Realms...a very small PIECE of Forgotten Realms.
Waterdeep, Baldur's Gate or the Dalelands are the most obvious choices. For what little I know about the realms, I think Cormyr would be cool too. Anyway, my things that immediately spring to mind are along these lines:
1) Defcon 1's comment bares repeating, it needs to start small. ESPECIALLY if it is going to be set in the realms. FR is a huuuuge place. You will lose EVERYone in a flood of strange place names. You need to introduce 1 or 2. When we've been there...tell us about 3 and 4. Adventure/climax in 5 and return to 1.
2) SHOW US them. One of several reasons audiences oo'd and ah'd at LotR was the SETTING! The places, the world, seemed REAL. The details of Bag End. Rivendell. Orthanc. The landscapes, in general. Doesn't have to be hugely in depth everywhere (e.g. DON'T take us somewhere and have the wizard character narrate us the place's 1,000 years history and 20 irrelevant person and place names). We just need snapshots...but it can't look like snapshots of the prop closet...it has to look like snap shots of someone's actual home, neighborhood, nation and world!
3) Diverse party. There has to be 1 mage in there (studying spellbook and all. Not some pouch of sparkly powder!). An elf would seem essential...he can have and use a bow...sparingly...but can't be Legolas. I'd say give us a BECM style "elfin chain" clad warrior-spellcaster. As would a dwarf...I'd go thief or full on "shining knight" paladin to not just be a Gimli-fighter clone. [Real] Halfling's are, no doubt, off limits. DO NOT give us halflings wearing shoes. Just leave them out. 3 or 4 humans of a few different skin tones. Two warrior types (minimum). DON'T shy away from the fantasy pantheon/religious elements of the world. We want the setting to seem real...make it a real world. FR is a pantheistic world. USE IT! Have a cleric be an actual religious-devoted cleric! The party must, at its base, contain 1 each Fighter, Mage, Cleric and Thief. Then I, personally, would go with a secondary and tertiary fighter type (if that includes a knight/paladin or ranger/barbarian/outdoorsy guy/gal or a elf ftr/mu, so be it). Introduce others for parts of the film, situationally and/or from film to film as necessary (like a powerful druid they cross paths with who helps them). But again, SIMPLE! Diverse, but
easily grokked character archetypes! Just because its a D&D movie doesn't mean the party needs to be sporting someone identifiable as Tiefling Warlock, Dragon Sorcerer, "Battlemaster-base with levels in Barbarian and Assassin" Warlord, or Elemental Monk.
4) Along those lines...The characters need to be CHARACTERS first, "D&D characters" second. Let the gamers sit in the theatre, loving the secondary warrior character "Sir Alderik, Knight of Cormyr" and go home (or, hell, nowadays from their phones in the effing theatre!) to get on the internet to posit and argue if we'll see a "knight" class in UA or is he just a well-raised Fighter with good manners and a code of conduct...or a Paladin...or a Cavalier (he had that scene on his horse!) or or or... But don't introduce "To aid your quest, we send you our nephew, Sir Alderik, Devoted Paladin of Bahamut the Great Platinum Dragon."...who happens to be half-elvish. Don't do that. Yes. D&D gamers will know exactly what you're saying. Anyone else will find it cumbersome and irritating and think through the rest of the film that the guy's name is Bahamut or he's a half-dragon or something.
5) To that point, and it goes for the characters, what I said about the places, and points back to #1 in general, "starting small": don't go over the top with fantastic names. You need to have conversation seem natural and so, drop locations and people in the world, as easy/natural in conversation. But stay away from just mentioning places and people (that no one or very few will have any familiarity with) until/unless that place or person is appearing on the screen. Keep us IN the setting, don't drown us under details no one will have reference for.
6) MAPS! Yes, LotR had the best and the FR ones probably should look remarkably similar to them (as for production and materials used to make them). But also Raiders series. The Mummy, I believe. Casablanca? I'm sure there are others. If there is going to be travel, even within a limited area, that we don't necessarily need to follow on an hourly/daily basis, again, SHOW us. There should be a map so we can say "or they're not just going to the Burial Mounds of What. They're going SOUTH, what appears to be some [great or little] distance from their Village of Where...and what's that clump of trees to the east there that says "Darkwoods" or that river...where's that go?" It helps with the making the world "real" for the audience.
7) I'll echo the "in jokes/terms/characters", but not many and not in a hokey/pun-ny way. But certain things that, say, go to he detail of the world...they don't matter for the film (or won't for this first film) but anyone who is "in the know" about D&D or FR will say "Hey! I know about that!" The Marvel movie chains and DC tv series have been excelling at these of late.
8) Have the party start/meet/re-meet/get details of your adventure/whatever at a tavern. A simple trope...nearly ignored in today's games but a welcoming introduction place. Who in the audience isn't going to understand meeting/gathering with friends at a bar or restaurant? Easily done, easy to make look/feel authentic, "grounds" the characters as "normal people doing what normal people do in this world"...and a nice nod to the D&D GAME (as opposed to the FR setting, specifically) for fans.
9)...Do I have a 9? I thought I had a 9...hmm...maybe after more coffee...but chew on these 8 for now.
[and just as an aside, not even dignifying it with a number, NO DROW! NONE! NO DRIZZT! NO DROW! Not in the first movie, anyway. You're introducing places and people, nations and at least 3 races (human, elf, dwarf) probably with other fantasy looking races in background/crowd scenes in large settlements...you don't need to show or mention drow! Leave them in the Underdark. Leave them mysterious. Leave them a "more frightening unspoken evil foe" for some future film. Drizzt is a FAR too divisive figure to try to incorporate in an initial offering.]
OH! Just remembered 9...
9) STAY AWAY FROM THE MACGUFFINS! You've tried it...Orb of Dragonkind anyone?...How'd that go for ya? If you don't want everyone to say "This is a shittypoor man's LotR ripoff!" You can't have them chasing after or seeking out or using or protecting from evil hands the "Uber Powerful Thing of Stuff!" You just can't. Borrowing from the Dragonlance storyline might prove useful here. Make it about an invasion or a war between nations or a magical mishap or something else...and, as mentioined by a few, REALLY make it about the ADVENTURE in the wilderness and "DUNGEON"!...but it CAN NOT be about the all powerful item!
EDIT: NOW more coffee.

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