This isn't a prediction more just what I'm sure will turn into a rambling idea.
The idea of balance isn't really present in the OT, so we can just skip that. It's really brought up in the PT, most famously in the "prophecy of the one who will bring balance to the force." To the Jedi it seems balance means that there is no real dark side presence in the galaxy. Basically the dark side if a perversion of the Force and any dark side means imbalance. This is somewhat why they are so worried about Anakin. If the Force has been in balance and this kid is supposed to bring balance then obviously that means it will soon be brought out of balance.
Now lets fast forward to TLJ when balance is really brought up again. There are a couple of instances that I want to bring up.
1-Snoke when he says "darkness rises and light to meet it". Then goes on to say he thought it would be Luke. This seems to show that he expected the light to combat the dark. That the 2 sides are in a constant conflict. Whenever darkness rises, light will also. And perhaps vice versa.
2-When Rey is reaching out and says she sees life, death, death feeding life etc. Luke responds something like "powerful light, powerful dark..balance." However this seems at least somewhat out of line with what the Jedi had believed. They seemed to believe that there could be no darkness if balance were to be achieved. Luke seemingly says something different. Now it could just be some throwaway line Luke says, but maybe not.
What if between ROTJ and when he started to train Ben that Luke had come to the realization that balance cannot be achieved by banishing the darkness. That if you do that the darkness will just rise again, then light will fight it. One side will win and then the other will rise to fight it again. A never ending cycle of conflict between light and dark.
Luke comes to the conclusion that the 2 sides have to co-exist for their to be lasting balance. That in order to bring balance and to be a true master of the Force that one has to master both sides of it. Now out of all his students who would Luke likely try to do this with? His nephew with the mighty Skywalker blood, Ben Solo. So Luke actually begins to attempt to push Ben down this path. Ofcourse Ben cannot handle it and falls to darkness. He cannot achieve the balance. Because although to achieve lasting balance one has to master both sides of the Force attempting to do so is fraught with danger. This could play into Luke's immense guilt over Ben. Not only did he fail as his teacher but his new teaching method basically is what started Ben's fall.
Now fast forward to IX. Rey is trying to lead the Resistance, defeat the darkness. However Luke knows any victory over Kylo will be temporary. Darkness will just rise again and the conflict will continue unabated. Luke returns as a ghost and tells Rey this info. Basically "ok, you can blow up the First Order and kill Kylo but darkness will rise again and in a generation there will just be another war. You cannot really win this way." Rey is obviously frightened of learning the darkside but she understands Luke and begins down this path to master both sides of the Force.
Thing is Luke don't know the darkside, so how can he teach it? This is why Rey seeks out Palpatine's spirit/ghost/whatever. Not to defeat him as the final villain but to learn from him. Ofcourse this won't go exactly as she'd like and she teeters on the darkness overwhelming her. In the end though she is able to harness the dark side and master it to the point that it does not consume her. Inside her the darkness and the light exist together in balance, so the 2 sides no longer end up engaged in some unending conflict.
This makes her the most powerful Force user ever and allows her to defeat the First Order and Kylo Ren (who plays a major role in this and himself perhaps attempts to master both sides of the Force but fails as he just cannot overcome the darkness).
Then at the end she finds a new student(s) but she is not training them to be Jedi (who only dealt in light) or Sith (who only dealt in darkness) but to be something new: Skywalkers.