Luke won’t go to the dark side, but I reckon he could be in exile somewhere, consumed with (or trying to balance) both sides of the force. He’s a Skywalker, his dad would have been the most powerful force user ever if he hadn’t turned into a cyborg. Imagine what Luke would be capable of!
Luke is wigging out in a cave somewhere, being light and dark at the same time. That’s why the force is quiet i.e. not awake, when this movie opens. Something happens to change that balance and it all kicks off.
I also bet that someone loses an arm, and either Han or Luke says ‘I’m getting too old for this.’