Yeah after hearing a bit more (particularly that Trystane was described as the heir to Dorne) I am convinced Arriane Martell could be cut.
(Potential unmarked minor spoilers could follow - not of any major events though, just some of my opinions on new characters and stories)
I'm not too bothered about losing her as a character, but if it's true I think it's a strange decision. It doesn't seem like the natural cutting down on characters (which I'm all for). The Dorne storyline (and the Greyjoy storyline funnily enough) is told in a rather unfocused way through 3 POV characters, but Arriane Martell is basically the main POV character for it in the books so far. The Sand Snakes are basically there to add colour and Trystane is like the Rickon of Dorne - yet they cast Trystane and 3 Sand Snakes.
My guess is the reason behind it is they wanted to add as many of Oberyn Martell's daughters as possible to cash in on the Oberyn love. Obara Sand will probably fulfill Arriane Martell's role in the story.
I understand why they'd do that, but it disappoints me for a couple of reasons: 1. The different inheritance laws (gender neutral) make Dorne a bit different and interesting (and may be important for the plot), and having the heir to Dorne be a woman does a better job of making that point than repeated exposition. 2. It raises doubts about them referencing any marriage alliances or planning future ones, which I thought were important for Doran's plans and the Dorne story in general. Dorne may not look down on bastards, but that doesn't mean they are as worthy a match as the heir to Dorne for someone like Viserys, or (f)Aegon.
I'm all for the show making changes and cuts, and seeing the show's changes to A Feast For Crows and A Dance With Dragons (which I think have some awfully meandering storylines which are in need of the focusing and tightening the TV show would bring) was one of the main things I was looking forward to. But I think I may need to recalibrate my expectations in terms of how different things will be, especially with the 7 seasons plan. In season 2 and 3, a storyline like Arya getting captured and then wandering around in the Riverlands was distilled to the essentials and received an awesome show-only addition (interactions with Tywin), and I was looking forward to seeing some storylines from books 4 and 5 get the same treatment. But now I think that any storyline that doesn't serve the endgame or complicates things unnecessarily will get completely cut (Quentyn, Aegon, possibly the Greyjoys, which is a shame since it's one of my favourite from books 4 and 5) and the existing stories will have a lot more differences / additions, and be a lot quicker paced.
Anyway, whatever differences may be coming I'm looking forward to it greatly. I love the intricacy and depth of the books which obviously a TV show can't match, but I think the show deals with the essential story and characters even better.