Everyone's seeming to come at this in terms of selecting the most likely narrative tropes for each of the roles the characters were filling previously, but Rian Johnson doesn't write like that.
What about their motivations?
Rey wants to understand her power and her role in the world. She's not driven by a desire to defeat the final boss or restore peace to the galaxy, particularly. She doesn't have any vendettas. Kylo's been trying to capture her, and he killed Han which Rey isn't too keen on, but she's not afraid of Kylo anymore. She lit him up. More so she's looking for meaning, identity. That's not a clearly light or dark side motivation, now, is it?
Luke's cynical, isolated, broken by Kylo's turn, and seems scared shitless of Rey's power and extremely reluctant to be the one to cultivate it. Snoke, on the other hand...
Kylo killed his dad, but is he willing to kill his mom? Maybe not. Han wasn't necessarily much of a father figure. Leia, though, seemed to be there, and seemed to care about him all the way. Kylo's something of a wild card. He's certainly emotionally unstable, and has a lot of murdering under his belt, but he's still internally conflicted. If Kylo fails to pull the trigger, is he still on terms with Snoke? Does he still want to be?
Finn has a strong moral compass, bravery to spare, and cares for Rey. He looks to be knee-deep in bloody Resistance operations, and as a traitor to the First Order his war is clear. When he eventually crosses paths with Rey again, who has little vested interest in the the political situation and could have a more willing mentor in Snoke than Luke, will they still be on the same side? What would the implications be for both Rey and Finn to reunite under those circumstances? What if Rey's parentage is more Palpatine than Skywalker, and her identity has been suppressed by Luke, maybe ending up in exile on Jakku by his hand in the first place?
What's the darkest timeline, then? Rey being trained by Snoke, Kylo's allegiance falling into question after sparing Leia and then being tossed aside for the latest and greatest when Rey joins up, leaving Kylo salty as hell and potentially without a master. Finn wants Snoke defeated for the sake of the Resistance and the fall of the First Order, and wants Rey on Team Finn and breathing, but Rey wants to know the full extent of her RAW, ULTIMATE POWER and the truth about her heritage.
Finn can't confront Snoke and Rey alone. An unlikely equalizer, back on his prescription mood stabilizers and returning calls to his Space Therapist, however, shows up and extends his hand...
