At Star Wars Celebration, a new movie set 15 years after the events of The Skywalker Saga was announced. The Sequel Trilogy did have a number of arcs that can be followed up on in this New Jedi Order era. Finn's is one of them, who's ex-Stormtrooper arc had a lot of potential, but was never fully realized unlike Mayfeld's in The Mandalorian. Finn could make a good Grandmaster, as he knows what it means to have a cause to believe in.
In the Road to The Force Awakens, the New Republic quickly started demilitarize the galaxy's militaries. However, and as the Rebellion should know, the smallest military coalition can topple a galaxy-wide government. Which again happend with the First Order through that mistake. As such, arcs playing out off-screen can feel like they don't make sense. The same happened with Luke, who showed up as a completely different person in The Last Jedi after Return of the Jedi.
Left Behind
An arc that can be followed up on is Rey's. In each of the Sequel movies, Rey found a parent figure in Han, Luke and Leia respectively. Only each time she had build up a connection with each of them, they instead turned to Ben in an attempt to bring him back to the light and abandon Rey. Eventually, this meant that Rey ended in the same place she started: alone on a sand planet without a parent-figure.
Just like how the Originals and Prequels had an arc of Anakin overcoming his parent-figure issues, the same can be done with Rey. The movie can start out with Rey having a kid, or maybe even Ben's kid, but she keeps abandoning the kid as a result of having gone through the same herself. By the end of the movie, Rey has to be the parent she never had and actually be there so the kid doesn't end up alone.
The Force Monad, Dyad and Triad
The sequel trilogy introduced the concept of the Force Dyad. This force-bond fits in well with the earlier introduced concepts of the Force Monad, better known as the Chosen One, and the Force Triad, also known as the Father, the Son and the Daughter on Mortis. With the Mortis Triad gone, a new triumvirate had to take it's place. With Anakin as the Force Monad, the roles of the Son (Ben) and Daughter (Rey) had to be filled eventually.
With Ben growing in the Dark Side, the Force corrected this by putting Rey into the Light Side of the Traid and resulting in her sudden Force powers in The Force Awakens. With both the Dark and Light sides now filled, the balance was restored. In the new movie however, Ben was returned to the Light. In order to keep the balance the Force could expel Rey from the Triad, resulting in Rey losing her Force powers again. As such, the movie can focus on a quest to restore those powers.
The Rule of One
The Rise of Skywalker introduced the Sith Eternal and the resurrection of Palpatine, which was later expanded on with Project Necromancer in The Mandalorian. The Sith Eternal worshipped Palpatine as if he were the only true Sith. This is similar to the Rule of One, where devotees swore blind obedience and served under the will of one Dark Lord. In Legends, Krayt achieved absolute power by prolonging his life, ensuring he stayed in power. This is not unlike what Project Necromancer aims to do as well.
It can be argued that the Force lost it balance when Sidious and Plagueis originally figured out the secrets to eternal life. Although the secrets were lost, Project Necromancer aimed to replicate them. As seen in The Rise of Skywalker these attempts eventually failed, but the next movie can pick up on this thread with the Sith Eternal keep trying. Possibly even under the leadership of a new Dark Lord like Krayt.