The thing that I'd really love to see adapted for the screen - although the length of it would probably mean that it needs to be a succession of mini-series (say about six hour-long episodes per book, roughly), is Julian May's Saga of the Exiles, Intervention, and Galactic Milieu Trilogy. Ever since I first read them I've wanted to see them adapted for the screen.
Also, a similar series of adaptations of Clive Cussler's books, rendered as faithfully as possible so as not to hack him off (he does have a habit of finding adaptations of his books offensively bad; in the case of Raise the Titanic I think that's a bit unfair, but in the case of Sahara I couldn't possibly comment), but with actual realistic non-clunky dialogue. Probably starting from about the point his books starting pushing the 500-page mark (which IIRC is Cyclops, although I wouldn't say no to an adaptation of Night Probe).
In neither case have I given any thought as to who should play the characters, though.