Board Thread:Spoilers!/@comment-26443838-20150920015305/@comment-5899967-20150921001948

It sounds likely because it's not something that can be solved in a single arc, especially not in 10 episodes.

The real evidence is in the synopsis for season 5, where it says the citizens of SB have to deal with Emma's transition as the Dark One, which makes it sound like it's just the beginning of her DO time.