Okay, I know some folks are thinking with Ellaria and the Sand Snakes killing Doran and Trystane that it feels like all the Dorne stuff from last season feels like a waste, but I think there's a possibility we're not considering.
What if, on the show at least, Doran Martell and Varys have been working together this entire time, with the goal of putting a Targaryean on the throne? In this case, it happens to Daenerys. We all know why Varys wants this, but for Doran, it's revenge against the Lannisters pure and simple. But of course Doran, unlike Oberyon, is far more prudent, less direct, and patient-- qualities of someone who thinks long term. Which is what Varys has been doing in King's Landing all this time, helping to create the conditions in which the Lannisters would self-destruct, allowing for Daenerys to come in and take over. If Doran and Varys were in cahoots, then Doran would know all about this, hence why he seems so "passive" even over the death of Oberyon.
However, with Varys out of the city, he needs someone else the finish the job. So he agrees with Trystane betrothal to Myrcella because that puts him right inside the Lannister household and on the Small Council, where he could easily act of Doran's spy. But he also has to have somebody in King's Landing to turn the people against the Lannisters so that when Dany arrives with whatever army she has, combined with the army Dorne's, and with King's Landing in disarray, the people would welcome them as liberators. And who could Doran send to cause such dissent against the current royal house?
How about the High Sparrow?
But of course, Ellaria didn't know about any of this, so her and the Sand Snakes killing Myrcella, Doran, and Trystane, they've now potentially jeopardized years worth of careful planning. Just a theory, mind you.
