A/N: A possible theory of what could happen in Episode IX, given what has been introduced in Star Wars canon so far. This is one way I think the Sequel Trilogy can wrap itself up nicely. The Force Awakens the Last Jedi, Rey and Ren.
Rey continues her studies in the Force. Luke's former students, the Knights of Ren track her down and try to kill her. However, she has grown so powerful, that she wipes them all out, leaving Kylo the last of his kind.
When Kylo learns of this, he tries to avenge his fallen brethren on Rey. He and Rey have an angry Force Skype session where accusations fly.
In the middle of their conversation, an unexpected third person shows up - Anakin Skywalker! He tells them that he has been watching the Galaxy closely, and both of them in particular, in the 30 years since his death on Endor.
Anakin explains that contrary to Snoke's claims, it was in fact he who had connected Rey and Kylo through the Force. It was he who had brought his old lightsaber, through a series of "coincidences", into the hands of Maz Kanata. It was he who had stirred up Ben Solo's interest in Darth Vader. He did all this because both of their destinies are intertwined with his.
Anakin teleports them to Mortis, and shows them the throne which he now occupies as the Father. He informs Rey and Kylo that he wants them to be the new Daughter and Son, respectively.
He shows them an alternate timeline where he did not become Darth Vader. Padmé still died due to complications in childbirth (that was always going to happen). Chancellor Palpatine found another apprentice and wiped out the Jedi using the clones. Heartbroken, Anakin raised his children in hiding, with help from Obi-Wan and Yoda.
Unfortunately, the Emperor found Luke, captured him and turned him to the Dark Side. Leia remained a Jedi. Eventually, an insane Luke, now known as Darth Jocular, teamed up with his father and sister, and the three Skywalkers took down Sidious together, with Anakin delivering the killing blow.
In the aftermath, Luke decided to remain on the Dark, but vowed never to take an apprentice as he wanted the power of the Dark Side for himself. With both Jedi and Sith Orders reduced to just one member, balance had returned to the Force and the cycle could begin anew. Ultimately, Anakin, Luke and Leia took up residence on Mortis as the new Father, Son and Daughter.
In reality, events did not play out that way. Sidious and Vader both perished in one day, making the Sith extinct; while Luke Skywalker set out to rebuild the Jedi Order. The necessary Mortis Succession was thus delayed for a generation, and the Force set out to put things back on track.
As a result, the Dark Side chose Ben Solo, Vader's grandson to become the new Son; while the Light Side raised up a nobody (Rey) to become the new Daughter. Why Rey? Because the Force knew that she and Ben would meet and develop feelings for one another, and that this would bind them together in a state of necessary tension and balance. It was the same phenomenon that brought Anakin and Padmé together, and which brought Han and Leia together: the union of one Skywalker and one ordinary person.
After that, the Force ensured that only one Jedi and one Dark Sider remained in the galaxy: Rey and Kylo. This was why Luke's temple was destroyed, and also why Snoke and all the Knights of Ren perished. There can be only two until the vacancies on Mortis are filled. Once there is a new Son and a new Daughter, Force users like "Broom Boy" Temiri Blagg will be able to flourish once again.
Rey and Kylo accept their destinies, and set their affairs in order. Kylo hands over the leadership of the First Order to General Hux, while Rey appears to Finn, thanking him for their friendship and telling him she will always be with him. On Mortis, Ren and Rey take their places alongside the Chosen One, keeping a watchful eye over the Galaxy forever.
Meanwhile, General Leia and the Resistance form a new Rebellion. They win some major victories, but Leia suffers an unexpected heart attack during one battle and passes away. The First Order uses Leia's death as propaganda. Poe Dameron becomes the new Rebel leader; but with the sudden natural death of their Princess, the New Rebellion is in danger of becoming demoralised. Rose regrets that Leia had no other children besides Ben Solo, who could lead them. Hearing this, Finn comes up with a radical idea: to clone Leia and have the girl become the new icon of the Rebellion. Happily, one of the Rebels is a Kaminoan and vows to make Finn's idea a reality.
Encouraged, the New Rebellion soldiers on. Unfortunately, DJ (who had infiltrated the Rebellion) informs Hux of the Rebels' plans, and Hux orders that Kamino be annihilated. However, without Starkiller Base, the First Order is forced to rely on Star Destroyers and other means. With help from an elderly Boba Fett (who had taken up residence on his home world once more), Dameron gains the support of a number of arms dealers, and the New Rebellion successfully defends Kamino with their new weapons. Desperately, Hux orders his men to use the lightspeed kamikaze. Viewing events unfold far away on Mortis, Anakin mind tricks Hux into blowing up himself and his whole armada instead.
Leia's "daughter" is born, amid much rejoicing. Finn names her after Rey, and vows to raise her as their new Queen. The New Rebellion reorganises itself into a constitutional monarchy modeled after Alderaan and Naboo, which is christened the Sky Kingdom in memory of Luke.
The sequel trilogy ends with there being two power blocs in the galaxy: The First Order and the Sky Kingdom. The Star Wars have only just begun.