Dorne?

Except this leaves Trystane's fate at severe risk don't you think? Doran's not going to risk his son and heir unless he's absolutely certain he can win the Iron Throne. There seems to be two possible scenarios that are valid:

1. Fire and Blood's theory where they send the sandsnakes to poison/assassinate Tommen and place Myrcella on the throne with Trystane as her king consort thus returning the Martells to the stronger leadership position they used to have when Elia was married to Rhaegar.

2. Doran reveals he's working with Varys to bring Daenarys to Westeros and place Trystane in her council or even as her hand if he doesn't have any idea Tyrion just took that gig.

I agree, it's a risk he's taking and I was surprised when he said that's what he's going to do considering the bad blood between the Martell's and Lannisters right now so soon on the tail of Oberyn's death. But with Jaime there, I'm guessing that's where the idea he's going to make Jaime swear fealty to him and make him swear to protect Trystane as equally as he would protect Myrcella. Jaime knows Myrcella's in love with Trystane so it's more likely he'll go along with that too.

I think I misread your original idea because it's basically the same as Fire and Blood's idea to have Tommen assassinated by the sandsnakes but instead Varys does it. This could be done either way but will make the Martells look very suspect immediately. With the High Sparrow in power, there's no way they wouldn't be questioned for another king's murder.

If Doran is working with Varys to ally and provide a soft landing for Dany, it would be safer to keep Trystane and Myrcella in Dorne but I think I see why he's sending them there, as you said - to start putting in motion alliances there for her to move forward.

Queen in the North Sansa Stark, the First of her Name, True Mother of Dragons

At this point, I don't care what happens in Dorne in the finale as long as SOMETHING happens. Jessica Henwick seemed like she was saying that shit would go down, and I am 100% in favour of that. Myrcella dies? Awesome. Bronn dies? Fantastic. The Sand Snakes are all executed? Perfect. Just don't leave me with a pointless storyline that went nowhere.

Maybe one of the Sand Snakes tries to kill Myrcella, and Trystane gets killed instead? I don't know I'm so annoyed with the underuse of Doran and Areo

I'm kinda gratified to learn the three eldest Sand Snakes really are pointless, since I've hated them since their book intro. Maybe they actually do shit relevant to the plot in Winds/s6 but that doesn't justify hyping them in s5 or wasting Indira Varma by making her Mama Snake. Why not wait until the Vengeance, Justice, Fire and Blood speech or early s6 to introduce when plenty of better characters were not shown on the show until absolutely necessary?

Yeah edges out Qarth for me as the worst storyline they've done

If they wanted to kill myrcella this was easily achieved off screen.

If Doran had no big plan reveal than this was a waste of time. Myrcella was chess piece she was the reason to go to Dorne and be introduced to a variety of characters. They were supposed to use her to get what they want- revenge for Elia. To bring down the lannister legacy. How do you suppose this happens now?

Thus no point to Dorne.

I really enjoyed Dorne. Nice seeing a new location after 5 years and it was definitely a beautiful looking place. It also served a huge plot point, so yeah, kudos to D&D.