Here’s just a wild idea, throwing it out there just to say it: What if we combine ‘Rey killed her parents’ with ‘Rey’s a nobody with no Force power’? The idea would center around the Sith dagger, the only unequivocal connection between Palpatine and Rey’s past.
Rey’s parents travel to Jakku, destitute and needing money. They agree to sell Rey to Unkar Plutt and immediately spend this money on booze. That night they party and pass out drunk. Rey, meanwhile, is wandering forlornly around the desolate settlement and has been drawn to something in a junk pile: a mysterious dagger. It calls out to her, and she grabs it from the pile. She approaches her parent’s ship with the dagger in hand, waking up the next morning with her hands covered in blood. She is found by a scavenger named Ochi, who takes the dagger from her and takes her back to Unkar Plutt. Ochi relinquishes her, but pockets the dagger. As Rey is being washed up, she hears the sound of her parent’s ship taking off, and she races outside to see it streaking up into the sky. She calls out to her parents to wait for her, but the ship disappears into the sky.
The dagger, it turns out, is a Sith artifact that points the way to Exegol, but only for those who are in tune with its dark power. Ochi gradually fell to its power, but nobody had ever used the dagger more readily than Rey. By this act she proved herself worthy of joining the cult of the Sith Eternal.
That would be a cool idea for a rewrite, but I don’t see any way it could possibly be pulled off in a fanedit. It would need tons of new footage that we literally don’t have.
As of right now, I’m leaning more towards the “ancient prophecy” angle as being the reason why Rey feels like she’s destined to turn evil. Maybe there could be a prophecy that Rey is destined to “destroy the Jedi”, only for that to actually mean that she’ll create a new order of Force users that rejects the old dogma? I don’t know, I’m just spitballing.