Have you read the original books? The original Dune books aren't exactly the stuff exciting TV is made of in the first place, especially after the first book. They are very philosophical and reflective with a Lovecraftian horror for a protagonist. It's more like Ayn Rand than Star Wars or Game of Thrones. Deviation from the original storyline is probably for the best, just because you don't have to follow the script since fans won't really have the same level of expectations. They can aim for something a bit different and capture a different aspect of the Duniverse than what the movies focus on.