As I said in another thread, I don’t think Regina will become the Evil Queen again by choice. I highly suspect that she is somehow given Jekyll’s potion, and turns into the Evil Queen against her own will. I think the writers miss writing certain characters when they were evil, and can’t help but figure out ways to do so again in the present. We’ve seen them do so with Rumple countless times, and we even saw them do it with Hook at the end of 5A. However, they want to have their cake and eat it too. By using Jekyll’s potion, they aren’t necessarily going against Regina’s character development and long term redemption. This would be like the Spell of Shattered Sight again, but probably more long term. She could also transform back and forth, which could add a new dimension to this.
That seems fairly likely. Not sure if I like it…but it seems likely. Rumple’s been doing it nonstop, Hook became the DO, so now it’s Regina’s turn.