It's been a while since I did a piece of this ilk: a rundown of some SF books that haven't been filmed, and that aren't likely candidates for same, but which I would love to see nevertheless.
The Foundation Trilogy. Isaac Asimov's series about a future galactic civilization imperiled by internal collapse has actually never been a good candidate for being filmed, in big part because much of the "action" is just people talking to each other about specific ideas. That hasn't stopped various hands from trying — although, last I heard, Roland Emmerich's version stalled in production, thank goodness. For that reason, it's a better candidate for a TV series than anything cinematic. The BBC actually did a pretty good radio play adaptation in the Seventies, which despite the bloopy-bleep sound effects is worth checking out.
The Shockwave Rider. John Brunner predicted not only the Internet but Wikileaks in this 1974 novel about a man who escapes from a human skunkworks and attempts to tear down the system that produced him. More approachable today than before for obvious reasons. One wonders how many more of its predictions will continue to come (unfortunately) true.
Stand On Zanzibar. Brunner's other masterwork, his "John Dos Passos / U.S.A" homage that intertwines three storylines about a massively overcrowded, unsustainable future. Dated in some parts but still incredibly prescient in others. Another great candidate for a TV series due to its length and complexity.
The Revolving Boy. I cite this one for its relative obscurity — I don't think I've met five other people who even know about it — its simplicity of conceit (it's about a kid who has an unfailing sense of direction), and how that idea is played out in a way that would work very well on a screen. And if done right it wouldn't even cost that much, either.
A Canticle for Leibowitz. This classic came up in discussion the other day, and if they can't make a live-action film of it, perhaps an impressionistic animated production would do the job. Hard to market, but perhaps no harder than something like Cloud Atlas — and this, in my opinion, has far more of the profundity that Atlas (as a book, anyway) wanted desperately to have but didn't know how to attain.