The Game of Thrones finale brought some major surprises - including the quiet arrival of the new Prince of Dorne. For a moment there, it looked like Westeros might actually get a democracy... but of course, that's not the Westerosi way. Instead, after Daenerys was killed by Jon, the new ruler of the Six Kingdoms was chosen by a council of Lords and Ladies, and it was a council that introduced a few new faces (as well as showcasing what happened to some fan-favorites).
Among the representatives of Westeros who met in the Dragonpit were the Stark siblings, Sam Tarly, Edmure Tully, Ser Brienne, Ser Davos, Gendry, Yara Greyjoy, and Robin Arryn - but they were joined by several unknowns. One is likely to be the long-awaited Howland Reed, and the others appear to be representatives of various other houses, but one is clearly the new Prince of Dorne - so who is this new Dornish leader?
There are various people who could be this latest Dornish Prince (as the title isn't a reference to the Royal family, but to the unique place Dorne occupies in the Seven Kingdoms - and the fact that the Targaryens struggled to conquer them) - but the chances are it's not a Martell. Although the books include more Martell heirs, such as Doran's son Quentyn or his cousin Manfrey, the show has made it clear that the only heir to the Martell house was Trystane, who was murdered along with his father.
However, several other noble houses of Dorne have been mentioned in the series, primarily when Dorne sent representatives to Joffrey's wedding, but not all of them have sons young enough to be this new Prince of Dorne. The most likely candidates are Perros Blackmont (son of the House of Blackmont, although with an older sister, he would not actually inherit the title of Lord Blackmont), Ser Ryon Allyrion (heir of his house, which is the only house other than the Martells who get a full appearance on the series, with his bastard son squiring for Oberyn), or Mors Manwoody (heir of his house, although a 'stony' Dornishman, which would likely make him much fairer than the new Prince). Ser Archibald Yronwood is another possibility - a member of the second most powerful house in Dorne, and one that has battled with the Martells for rule for years.
Of course, the events of Game of Thrones definitely ended by thumbing its nose at ideas of inheritance, heirs, and traditions, so it's also possible that this new Prince of Dorne is none of the obvious choices. He could well be a previously unmentioned Martell cousin, or the heir to another house - or someone else altogether, who was able to take power in the chaos left by the deaths of the Martells. He exists in the scene not because he is an important character in and of himself, but in order to show that Dorne has not been completely forgotten by Game of Thrones.
It's also interesting that although Dorne has always been fiercely independent and somewhat difficult for the monarchs of Westeros to control, this new Prince of Dorne didn't attempt to hold it as a separate kingdom, in the same way that Sansa separated the North. Presumably, this would have been too much to cram into the scene, and would have taken away from Sansa's victory for her Kingdom, even if it would have made more sense given Westerosi history. It's possible that fans may discover exactly who this Prince is in the coming weeks, but it's also possible that HBO didn't actually decide themselves, and simply made him an unnamed representative - leaving fans open to decide who they want to be ruling Dorne now that the wheel is broken.