Myrcella/Trystane. Because they deserved more than death, even if that's what they want to give Tywin and the Mountain.
Myrcella Lannister (for she will always see herself as one, unless she truly marries Trystane) had never thought she would have his cousins' respect. But after she gave birth to Nymeros, and gently guided him through reconciling Doran and Mellario, the seeds were sown.
From her first blood until her eighteenth nameday, Myrcella bore Trystane four children. Nymeros and less than a year later, his younger brother Mellarion. Melia and Myrra were twins and the last children she carried until Cersei, Tywin and Robert were coerced, threatened and tricked into allowing her to marry Trystane.
Obara, who visited from the Wall often as she could, and the oldest Sand Snake, grew fond of Myrcella because she had reasoned
"If it weren't for Tommen, Tyrion, and Jaime, the direct line would be a waste of Lannister flesh. No, better I give Trystane and Dorne the tools to bring them all down, and even if it takes until the children can talk and think, Dorne will have Lannister minds working on their victory."
Much as her father wanted to avenge aunt Elia, it had been agreed that not only did Sandor Clegane have the best chance, but also the most cause.
Sarella visited only once, and was pleased with Myrcella, who made sure her Desert Lions were taught everything they would ever need. The Lannister girl also listened to Sarella's recommendations about what needed doing for her cubs to grow strong.
Tyene, Nymeria, Elia, Obella, Loreza and Dorea had studied Myrcella curiously, the younger ones delighted that Myrcella played with them, the older ones interested in how well she fought for a pregnant girl.
Trystane was grateful that Myrcella loved him enough to give him tools to destroy her grandfather, but he hated seeing it happen. His blonde beauty had fought for hours to birth his babes every time, in so much pain that only his touch helped even a little.
His sons, Nymeros and Mellarion both came out quiet, black haired and red faced. But his darling girls, Melia and Myrra, were squalling bundles of gold. His sons received nestling snakes but his daughters only got lion cubs, and of course, the Desert Lion banner their mother created.
She had, in a fit of sentiment he disapproved of, sent a banner to the current Sand Snakes. It was a red viper, lunging on a yellow field. The Desert Lion banner was similar, a crouching red lion on a golden field, all details etched in black.
In that life they wouldn't live again, his cousin had killed him, and Ellaria had killed Myrcella. He only allowed Obara around because she loved Benjen as deeply as he loved Myrcella. Tyene and Nymeria he watched closely, never straying from Myrcella's side longer than it took to find the children.
The North had surprised him in their acceptance of certain Dornish customs. Robb Stark had waved it away and dismissed
"The Old Gods and their Way are reclaiming the North. A bastard is the product of the Sept, not the Children."
Obara had gone starry eyed, begging Benjen to tell her about all of it, and with a sigh, he had.
When he finally wed Myrcella, their children were in attendance, and Gods but he enjoyed Cersei and Tywin Lannisters' faces when Melia and Myrra ran for their mother, Myrcella murmuring to her daughters. His sons came to collect their sisters, and his parents' eyes twinkled. Uncle Oberyn had agreed that having more children- bastard and trueborn- than the Lannisters was one way to one up them. His own mother had birthed another son, to his fathers' delight, and Arianne was finally considering marriage. Ellaria cradled his ninth girl cousin with tender hands.
Although he knew he shouldn't, Trystane revelled in that his parents were even happier than Oberyn and Ellaria. It seemed like a peaceful kind of revenge, on his cousins and their mother who thought murdering Myrcella would solve anything.
He can't act on what was, much as he wants to throttle them. Instead, he trains in all matters. He attends every Peacemeet, converses with every intelligent member of the Winterfell household, plays Cyvasse and other strategy games. He writes to Quentyn, and Arianne, and Mother and Father and baby Dorion, plays with his children and teaches them all he knows.
Trystane watches Myrcella train their children with the expectation that the task she is supposed to set them will be long done before she ever has to. And it is. Jaime, despite being heartbroken at having to do it, sets up the assassination of his sister, and poisons his father. Tyrion arranges Joffrey quite nicely. Arya Stark sends Petyr Baelish to the Many-Faced God with a smile. Ramsey Bolton is long dead, as is Roose, Domeric installed in their place.
Nymeros chafes at being raised in Dorne after the wedding, trained from birth to fight for his people as he is. Mellarion, too, wishes to be useful. Melia and Myrra are training under Arya to be Spymasters for their aunt Arianne at Winterfell. Myrcella prods him into sending the boys back to Winterfell to assemble as part of the army backing Theon Greyjoy. He dislikes having all his children so far, but they are bastards, able to move freely if he but takes his eyes off them. So he reluctantly permits it, soon enough distracted by Myrcella's newest pregnancy.
Tommen ascends the throne, and after settling the debts as he promised, calls for the Dragonlords from the North. Robert is long dead, as is Stannis, Arianne complicit after being brought into the fold. Viserys across the sea might be opposition, but Tommen is clear in his proclamation
"The Targaryens' are Kings and Queens by right of Conquest. Let only the sanest be let back onto the throne, and they must swear by their relatives blood to step down if the realm demands it."
Dorne is granted back their kingdom 'so long as friendship is maintained with the other kingdoms in the name of peace', as are the North, the Stormlands, the Iron Islands, the lands of the West- they go to Tommen while Tyrion searches for Tysha- the Riverlands and Dragonstone.
The Iron Throne is dismantled, as the molten swords go to Gendry Baratheon, Smith King of the Stormlands, with a list of names and descriptions to be restored. Aegon and his dragon supervise and provide flame as needed, Renly continuing to rule in his nephews' name.
Jon Stark Targaryen and his wife Daenerys come from Winterfell on the backs of dragons, Sansa Stark Greyjoy and her husband Theon ride in on the first of their ships, Dorne arrives in a royal cruiser, the Riverlands send a grand envoy, as does the Westerlands.
The Crownlands, it is decided, will remain a royal gathering place, and a Lord and Lady Paramount will be appointed, as this will be where all the noble children in the realm will come to sit the Trials of Adulthood, a series of tests geared to help their home Maesters educate them without bias. When the Trials are not in session, it will be the seat of education in Westeros, Aegon decrees. Which means that Maesters would come to this place to learn how to teach, and the common children would learn reading, writing and sums, Daenerys finished firmly. As Winter Town already had a school in session, Aegon asked if the staff of the Lord Paramount could be put there to learn.
Oldtown was told to prepare more Maesters, and the People of the Forest and the Green Men to prepare more lady healers. Tobho Mott was told to put the word out to smiths that graduates of Maegors' would be permitted to search for jobs.
Myrcella could scarcely believe it. The better world they'd fought for was here. Arya's Spymaster apprentices hailed from all over the kingdoms, as did Gendry's smiths. Jon had taught many children the proper way to wield a sword, and Theon a bow. She herself had taught the others how to apply Cyvasse to life, and now…
Now the world could prosper.