Ray could go dark because he's been hanging out with Nora too long. There's always Ava too. Or we could get Rip coming back being evil now. Amaya had trouble controlling the totem beasts at the start of season 3. Maybe they take her over completely? Sara could go bad too. They didn't say if any of these Legends would permanently turn evil, just that they would. Maybe it could be for a few episodes or most of the season, but are rescued in the end. That could be the season arc.