I must say I frowned when I read some of the suggestions that struck me a bit in the direction of RP and / or fanfic (I have very little experience with either, so my concept of them may quite well be a little foggy ...
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First off, I would add nothing that is not covered by the books, appendices, UT or HoME - or the letters. I would also guess that a trilogy wouldn't be enough (the EEs were 3:38, 3:34 and 4:00 hours - my backside hurts just thinking about that, even with intermissions!
). JRRT's own split into six books has a certain attraction - but staying with Merry / Pippin, Aragorn / Legolas / Gimli and the returned Gandalf for such a duration, book three, before returning to Frodo / Sam / Gollum, book four, might really be stretching it in a movie format, so shorter back-and-forths would probably be necessary. And my gut feeling would probably mean two 90-minute films for each book. The chapter "The Shadow Of The Past" could become a fairly long prologue (at least one film) with added information from the appendices, or parts of the reports from some people in the Council Of Elrond.
I would also very much get rid of what I like to call the "dumbing down" of some characters. PJ's Faramir has to go! Faramir was an Aragorn Jr., wiser than even his father in some aspects, and never mind big - I'm going to say it! - dumb bro Boromir. Then the crap with then Ents, turning the Ent-moot's result on its head - out it goes. And Gimli is not a comic-relief character, Dwarves are the one people who can make even Orcs hesitate simply through their grimness! Then heroin-junkie Frodo - when the Hobbits get into a tight spot, they should show much more toughness!
For Elves, I must say my imagination is at sea how to display them. True, three generations like Galadriel - Elrond - Arwen should look pretty much the same to non-Elves, but not like smooth-faced teens or twens, certainly more mature. And there was one comment about Elrond being partially human - so why not receding hairline, E~S?
But who would film this? I've read a lot about Douglas Adams' struggle to get "The Hitchhiker's Guide To The Galaxy" onto the big screen, which didn't happen during his lifetime. As one biographer put it unkindly, but probably correctly, DA was never going to have the film he envisioned made by Hollywood, so it could only have been - and was - made after he was no longer around to be an impediment. Probably my ramblings about LoTR should be made would send all Hollywood types shrieking for the hills, screaming "that's H2G2 squared!!!"
More of the serious background and relationship development could be added in. In fact, one could make quite a good 'prequel' movie based on the story of Aragorn and Arwen. Not sure how the dynamic would work for Arwen while Aragorn was off being Thorongil. There is a whole tale in itself on the travels of Aragorn.
