I can think of lots of classic sf novels that seem better suited to film. CHILDHOOD'S END, MORE THAN HUMAN, THE DAY OF THE TRIFFIDS (again), HAVE SPACESUIT WILL TRAVEL, THE MOON IS A HARSH MISTRESS, DANCERS AT THE END OF TIME, LEST DARKNESS FALLS, THE CHRYSALIDS, GATHER DARKNESS, maybe something by Poul Anderson or William Gibson . . . .
I'm surprised that Niven's Ringworld isn't on the short list of big sf movies that someone wants to make. Epic scale, great potential visuals, CGI for the aliens, a little sex... all things Hollywood usually loves to do! The sequel novels drag in a lot more of the Known Space background that would take some explanation in follow-up movies. But probably doable, if the first movie goes over well.
Also, for a little military SF, something set in Gordon R Dickson's Dorsai universe might do well at the box office.
ETA: Greg just beat me to mentioning the Dorsai universe. And Witch World would also be great!
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