I don't think there's a scenario where she ends up evil on a permanent basis, but it'd be a great plot twist.
TFA - we meet the new kid, we all like her, and she seems to be the new hero.
TFA - we meed the new bad guy, kylo, who is a skywalker who went bad when training with luke...from what we know anyhow.
TLJ - we continue with Rey as our hero only to have her turn in the end to be the new bad guy
TLJ - Kylo realizes the error of his ways and comes back at the end to be redeemed,
leaving, E9 to be the team up of Luke/Kylo to fight Snoke/Rey for all the marbles.
Would be a very cool story, spins TFA in a new light, and still leaves multiple possibilities for 9.