The Silmarillion was heavy going. Maybe a collection of sepatarate stories that might be filmable. Might even produce watchable films, but the most watchable films don't come from big, thick books. They come from quite brief paperbacks.
The Unfinished Tales............anybody remember the throwaway line in Spaceballs, the one that mentioned "Spaceballs 2 - the search for more money"?..... I felt the UT was headed there.
Don't know if there is a film there, but I have long had a liking for E R Eddison's The Worm Ouroboros. Not just for something written in 1922, when a fountain pen was a high tech word processor, but a cracking story by any standard. Apart from the first chapter.