I think it will be the Tommen who dies. Doran's choice to send Jaime and Myrcella and his own son back makes absolutelynosense, none, without some alternate plan. He's essentially giving up a hostage, and in turn giving the Lannisters a hostage of their own. However, a lot of stuff makes sense when you look at it as if Doran is trying to put Trystane on the throne. If Doran is sending the Sand Snakes to act as "personal guards" for Trystane, they will be allowed to get close to the king without suspicion because Tryastane will be on the small council. They can then poison him, which will open up Myrcella for the throne (Stannis would then have a stronger claim but he might be dead OR it's not like legal claims have mattered.) Trystane would then become king being Mcrella's husband, and Dorn would be the most influential land until Dany lands.
This would explain why Ellaria Sand swore her fealty and then magically became nice, and made peace with Jaime oddly right after, despite her capitulation being only skin deep from what we saw. If as assume Prince Doran shared the above plan for revenge with her, to steal the Iron throne and kill Tommen, and she was very happy after and was willing to make peace to hide the knife that was coming (And throw Jaime off guard so he wouldn't see the threat.)
I doubt it will be Jaime, due to the prophecy of him vs Undead Gregor, but it might be--we don't know if he dies from LSH, so this might just be an alternate end for him. But I doubt it, who knows though. I do think it will be an assassination.