I've wanted for decades to do an adaption of Faulkner's novella, "The Bear." But Faulkner doesn't adapt easily, and I think it would be a very hard sell. Same with Sherwood Anderson's "Winesburg, Ohio," though it might, with care, make a terrific limited series. But, again, the adaptation would be hard.
Here's one that I think would make a good film that wouldn't be hard to adapt, though: "The Voyage of the Narwhal" by Andrea Barrett, the story of a ship that sets sail for the Arctic in the mid-nineteenth century to look for a crew that disappeared a decade before. It would probably cost a lot to make, but it would be good one to see.
And this one: "Strait is the Gate," by Andre Gide. I'm surprised it's never been done. And another that I'm surprised hasn't already been done: "A Burnt-Out Case" by Graham Greene.
Also, I'd like to see an adaptation of W. H. Hudson's "Green Mansions" that does the wonderful novel justice. The 1959 adaptation is an abject failure, in my opinion.
Finally, "Stillwater," by William Weld (yes, that William Weld), a wonderful story about small towns in western Massachusetts being destroyed in the 1930s in order to build the Quabbin reservoir. Reminiscent of Kazan's WILD RIVER in some ways, but with different kinds of subplots. I think it could make a very interesting film.