Rey
I think one of the big things with Rey is her abandonment issues. So the way I envision it, she would keep pushing the idea that her parents must have been Jedi and abandoned her for a good reason, explaining why she is so proficient with the Force. When she's confronted with the truth, that her parents sold her for booze money and how she's only powerful because the Force is attempting to balance itself, it leaves her in a pretty low spot at the end. Her parents were nobodies and she meant nothing to them, so she feels like she's less than worthless. This ultimately gives her something to bounce back from in Episode 9.
Finn
I think instead of the whole Casino plot, Finn instead goes off to rescue a rebel held captive by the First Order who can help them on Snoke's ship. He goes with more than just Rose. During this whole plot, the reality of the situation is pressed upon him. You have stuff like people who remember the old Republic in a negative light, so the idea of a group trying to restore such a bloated and corrupt system are the bad guys to them. There's also how these people felt safe under the Empire, believing the information released by the Rebels/New Republic is propaganda. And there's the fact Finn was abducted and forced to fight as a Stormtrooper. He could be confronted with the face of a old friend, maybe even try to sway them to joining with him, before one of the rebels kill said friend like it was nothing (say Rose believed she was saving Finn). In the end, Finn is left conflicted about whether the Resistance are really as good as they think they are. First Order are still bastards stealing children, but the Rebels aren't innocent either.
Poe
This one is pretty easy. With his demotion, Holdo still doesn't tell him the plans. Instead she gives him orders and tasks to do in preparation, as benefiting of his demoted rank while talking with those who now outrank Poe. Poe fails to see the big picture, which leads to the mutiny. This resolves itself, and she also mentions she had suspicions of a FO spy so she kept her plans on a need to know basis. This gives Poe the push to become a more collected leader in the Future.
Humor
The appeal of the humor in the original trilogy was it wasn't just the jokes. It was more an extension of the character's personalities, with C3-PO being a coward or Han having to improvise. So, make the humor more character-centric, rather than everyone making quips.
Kylo Ren/Luke/Snoke
Luke tried to restore the Jedi Order following RotJ, making some adjustments based around how they fell. Stuff like not taking Force Sensitive children from their families. But due to his experiences on the Death Star II, he does ascribe to emotional control quite heavily. It was a major focus of his lessons in an attempt to preventing his students from falling to the Dark Side. However, this didn't always work out. Luke was very close to Ben Solo, which led to accusations by the other students that Luke played favorites. Ben refused to believe this.
The new Jedi tried to be of service to the New Republic. During a mission where they were sent meet with diplomats of a system that was considering joining the Republic, they were attacked. Ben was separated from the rest of the group and Luke sought him out. Unfortunately, this also meant Luke was unable to protect the rest of his students. A number of them died during the attack, including some close friends of Ben.
Ben was enraged at Luke saving his life over the rest of his friends, believing Luke didn't have the emotional control he always preached and did favor him due to their relation. In his rage, Ben destroyed the temple Luke used as his school and left with other disenfranchised students. Feeling himself unable to be the Master, Luke disbanded the school and went into hiding.
The negative emotions of the event had an affect on Ben, who tried to hide from his family. He was eventually found by Snoke, who told him to embarce his emotions turning Ben Solo into Kylo Ren.
The big reveal though is Snoke wasn't a Sith lord, he was a Jedi who went into hiding following Order 66. During this time, he observed the order established by Palpatine with a certain sense of admiration, since Jedi were supposed to uphold order. After the Emperor's death, Snoke engraciated himself with the remnants who would become the First Order, hoping to establish a new Empire founded on Jedi principles. Kylo Ren is supposed to be his enforcer, but his out of control emotions are actually meant to be his leash. This backfires when Kylo kills Snoke and decides to take over the First Order with Rey (believing it would be the Balanced thing to do) , like how Vader wanted to overthrow Palpatine with Luke.
The Force
I'm going to borrow from KOTOR here. The Dark Side and the Light Side are just two extremes, and when one has too much power the Force will act to try and balance it. This allowed small numbers of Sith to take out the Jedi Order, or for young untrained kids to have incredible Force potential to be champions of the Light. The fact that the Jedi and the Sith are the antithesis of each other in terms of teaching, yet both can use the Force without issue, suggests neither have a full grasp of what the Force is.
Rey takes the Jedi books from the island, but she also finds Sith books in the Dark Hole. Luke is the last of the Jedi and he does fall trying to bring Ben back to the Light, but the ending suggest Rey is going to develop her own philosophy of the Force based around understanding both the Dark and the Light. In essence, she's not going to balance the Force by being a powerful Light-Side user, instead her own approach will be balanced giving in to neither extreme.
