I can't believe no one has brought up Iain M. Banks yet. His Culture books provide nearly as limitless a selection of plot lines as El Neil's stuff does. Considering the Culture spans tens of thousands of years as a civilization, the possiblities are vast.
Ursula K. Le Guin would be another one. While Earthsea miniseries really didn't live up to it's potential, this was mostly because Ursula wasn't consulted on anything. The Left Hand of Darkness deserves at least it's own miniseries.
Dan Simmons' and his Hyperion Universe.
David Weber's Empire.
Anne McCaffery's Dragons of Pern.
Michael Moorecock's Eternal Champion.
Terry Brooks and his Shannara saga.
Asimov. Heinlein. Herbert. Clarke....
No... coming up with new ideas for mini-series, movies, or even regular seasonal series, isn't the problem. The problem is mediocrity. Polishing a turd until someone buys it.
How else do you explain Sci-fi movies like Sabertooth and Minotaur? How else do you explain a Sci-Fi channel with friggin' WRESTLING.
That's why I cringe every time I hear SCIFI is "adapting" a classic novel. They butchered "Riverworld," too.
I hated Riverworld, BTW. Some of the concepts were interesting but the characters...ech.