Balon Greyjoy, Lord of Iron Islands, captain of the Great Kraken, presently sitting on a Seaston chair (equivalent of Iron Throne in Iron Islands) sent his daughter Yara Greyjoy (in books her name is Asha Greyjoy) to rescue his son Theon Greyjoy captured by Ramsay Snow. She realized that her little brother is dead, he has turned into full- blown “Reek” (a name given to him by Ramsay). But then the entire Ironborn storyline was totally dropped off the radar in season 5, but now the show producers are hinting at a major comeback of Greyjoy family for season 6. It is now official that one of Yara’s uncles Euron Greyjoy has been cast for Game of Thrones season 6. He is a brother of Balon Greyjoy, captain of the ship called “Silence” crewed by mutes whose tongues were ripped out by him. He is one the deadliest pirates in the whole of Westeros.
The line of succession is a huge deal for House Greyjoy. Theon Greyjoy is a natural successor but he fell off the deep end, hence Yara is more deserving one to sit on Seaston chair. There is a strong possibility of a Kingsmoot ceremony hosted at Iron Islands where all the noblemen choose their next captain. It is much like “The Great Council” ceremony in Westeros where great Lords can jointly decide who should be the next King of the realm. Euron may be a formidable contender for the chair as he is more mature and respected in the eyes of local people. Theon is totally unfit to rule and Yara is a woman.
House Greyjoy still controls a vast amount of territory and resources. They can reunite under a new, powerful leader and march on Winterfell to avenge humiliation caused to Theon Greyjoy by Ramsay Bolton. Add to this what if Euron Greyjoy found out Dragon-horn or dragon binder in his voyage to Old-Valyria which could be used to control dragons, this could significantly shift the power structure and would overnight give rise to another player for Iron-throne.