I think the issue with the Dornish is that they're important to the story of the books and to the overall story of the show. You can't have an entire realm of the Seven Kingdoms that's literally never mentioned, unless you just...cut all of Dorne away and never, ever have it included. But that wouldn't make a ton of sense, given the backstory for the whole thing.
The Dornish are important as big Targaeryan supporters, which is relevant to both the book and the show. Where the show dropped the ball, though, was in devoting enough time to actually make the Dornish stuff have that point. And part of that you can lay at Martin's feet, since he draaaaaaaaagged out the reveal of Doran's plot in the books. I think the showrunners should've spent more time developing Doran earlier in the show, kind of like what they did with Theon. If you recall, Theon's story as Reek began in, what, Season 3? Yet none of that appears until book 5. He just...disappears from the storyline after the failed attack on Winterfell for an entire book (book 4). I think you don't even know Theon is Reek in the earliest chapters of Book 5, for that matter. Yet the show kept all of that stuff running contemporaneously with the rest of the narrative.
The real problems with the novel series -- which made for major issues with the show -- arose from Martin's decision to split Books 4 and 5 into two books, and make them so incredibly long and (in many cases) kinda pointless. There's a lot of stuff that happens, but we're STILL waiting for the payoff in the books. Meanwhile, the show outpaced Martin's writing, and started revealing the "big picture" immutable plot points. It also provided a lot of shortcuts that the books can't, because the narrative is so convoluted in the books.
Don't get me wrong. I love the books and the rich background they provide, but really, Martin's pace just doesn't jive with TV writing.
My guess is that the Dornish will play more of a role in the books, if only because their motives will be examined more closely, you'll spend more time with POV characters than we did on the show (like...any time at all), and their involvement in the war will be more impactful, but not necessarily in ways that tie into the show's overall thrust. I think the show is really cutting the narrative down to the bone this season, to seriously focus it and wrap it all up.
There's probably a good bit more involved in the Highgarden/Maergery plot with the High Sparrow than what the books will show. Bear in mind that, in the books, Cersei has literally only just walked through King's Landing naked. The Sept of Baelor still stands. The High Sparrow and Faith Militant are still a threat. Maergery is imprisoned. Loras has been horribly burned while trying to mount a seige (at Dragonstone, I think?). So the notion of "Aaaaand then we just blowed everyone up real good" isn't necessarily what'll happen. It may, certainly, but it may not. It might just be the showrunners cutting the cast back dramatically because, to hit the notes they want to hit in the time they have to hit them...they have to cut this stuff away. That doesn't mean it isn't important, even essential, but it does mean it may not be important or essential to the show's storylines.
With that in mind, I think it's worth considering what the show is concerned with:
- Dany conquering Westeros and claiming the Iron Throne.
- Jon fighting the Army of the Dead (and maybe discovering his heritage?).
- The Lannisters botching the job of ruling and being defeated. Including subplots of Jaime's love for Cersei, and Cersei's inability to rule and cavalier attitude towards...everyone and everything.
- Tyrion's ultimate resolution and place in Westeros' future.
Aaaand that's about it. Everything else is a subplot that feeds into those major plots. And if it isn't...it gets cut. So, the Dornish? Cut. The Tyrells? Defeated. The Greyjoys? Defeated and imprisoned. Theon's story may yet develop further (some kind of redemption at freeing Yara?), but it's not a CENTRAL focus of the story.
