It's sounds like Spaith was only involved in trimming down the original script. Kinda the opposite of his role in Prometheus. His earlier Prometheus drafts had some good bits though.
Not so sure about Dune becoming the next GoT, it's probably a bit too "weird" for most mainstream audiences, but you never know. Most likely it will perform similar to the previous two Dune TV series'.
WB probably have big plans if the first movie does well. Take for example Duncan Idaho, he's barely in the first book so casting Jason Momoa (someone that's currently very hot in Hollywood) seems like a waste unless you're planning to delve into the sequels where he becomes a main character.