Finn would be a story similar to Darth Revan. He was once a Jedi, turned Sith, killed a lot of Jedi's as a knight of ren because the Jedi killed his father mace windu, gets betrayed by his partner, nearly dies, loses his memory. The Sith knows how powerful he is and can't kill him so they manipulate him into being a storm trooper and their secret weapon, they could also tell when he's awaken. He has the powers of the Sith but he's not one. The reason why Kylo didn't kill him is because Snoke (Darth Plageis/Anakin Skywalkers father told him not too).
Rey is a reincarnation of the first user who's neither dark or light but actually more powerful possessing both sides. Her soul was transferred out of necessity from the whills (the creators of the force) to one of Luke's nearly dead students when the knights of ren attacked his camp to restore order.
Kylo is a Skywalker and one of the knights of ren who betrayed his uncle to be more like his grandfather and great grandfather. The knights of ren are a secret order of the most powerful Siths that were once Jedi's.
The whill reincarnated as Rey came down to restore order by bringing Ben back to the light side and informing him of the truth about Snoke/Plageis. That Plageis is dead and Anakin turned good before dying and the knights of ren are manipulating him using a deceased body.
The end of the last Jedi will convert Kylo to good and team him up with Rey and Finn (Jedi, Sith and Whill) against the knights of ren. Luke Skywalker was the mastermind behind the whole snoke plot but Darth Plageis has been dormant inside of him from an evil whill who is the creator of the dark side. Luke trained the knights of ren and only a whill (Rey) can remove the spirit of Plageis from him.
Edit: in the end, the team fights a giant force ghost of a whill.