They might not necessarily be villains, but would have the potential to really throw a wrench in the works. Of course we’ve already tread some of that path with the Greg and Tamara plotline. Maybe some characters who were actually personable and not shady.
I think that for the benefit of its people, Storybrooke needs to come to an end(1), because I see it as a metaphor for being halfway between childhood and adulthood. Growing up is traumatic and painful, but the alternatives are much worse; the town even has its own in-story metaphor for this, in the form of Peter Pan.
If I were writing the show, Emma’s ultimate role would be to help the townspeople grow up and move into the real world. Because she would be threatening the town’s idyllic stasis — its fairy tale — many would see their crown princess as the destroyer of their world.
By season 5b, villainous villains just aren’t effective threats anymore. Hades seems like he’ll be entertaining, but he’s not there to make us fear or think. He’s there to make us laugh. The show needs an antagonist, someone to move the story beyond cartoons and soap operas and make it relevant as a cautionary tale, the way fairy tales are meant to be.
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(1) Not saying “cancel the show”; that’s a different topic entirely.
Gaultheria's fanvids: http://youtube.com/sagethrasher