Euron (Theon's uncle) has taken over the Iron Fleet after killing Theon's dad, and he's going to make it clear he wants to be Cersei's king. He pledged fealty to her and ended up going to Essos to get the Golden Company to join her army. With these sellswords and his own fleet, Euron and Cersei are planning to let the Night King and the North take each other out so they can finish off the scraps.
It's as selfish a plan as any, but could be scuppered by Theon finally rallying the last supporters of his sister, Yara, to attack Euron and take back his kidnapped sister. Apart from being a revenge mission, it may severely deplete Euron's legion by the time he reaches Cersei.
THE NORTH'S SECRET WEAPONS
Brienne's allied with Jon now that she's fulfilled her promise of helping keep Arya alive, not to mention she's lost faith seeing Jaime still loyal to Cersei. After seeing Bronn also allied with Jaime, Podrick returned with Brienne to Winterfell, although he's back with his former mentor, Tyrion.
They'll be joined by Tormund and the Wildlings, Jon's other friends from the Night Watch, as well as the blacksmith Gendry who's making special weapons thanks to Sam and Gilly discovering the secret of obsidian (or "dragonglass") for them to kill the White Walkers.
Another wildcard could be Melisandra (the Red Woman), the exiled witch who brought Jon back to life, but who's also fated to die in Westeros. Perhaps she'll help him once again with some some more dark magic, acting as Jon's trump card.
Sandor (the Hound) picked Jon's side, hoping to exact revenge on his brother, the Mountain, for burning him in their youth. Of course, it's all in the interest of self-preservation, as the Hound doesn't see Westeros surviving the Night King's assault under Cersei's rule.
It'll be intriguing to see how he and Arya finish their rivalry, but that aside, what fans are waiting for most is to see him and his zombified brother finally engage in a battle to the death. The Hound's seeking atonement, while the Mountain simply wants to bask in blood and gore, amplified by the fact Qyburn turned him into an undead bodyguard.
Season 8 of Game of Thrones premieres April 14. The HBO drama stars Peter Dinklage as Tyrion Lannister, Nikolaj Coster-Waldau as Jaime Lannister, Lena Headey as Cersei Lannister, Emilia Clarke as Daenerys Targaryen, Sophie Turner as Sansa Stark, Maisie Williams as Arya Stark and Kit Harington as Jon Snow.