Applejack - Element of Balance
(Large image, so linking to DA page)
That actually sounds close to the evil Twilight I had in the Ravenloft module earlier. :D She might make a comeback since she wasn't slain...
But for Lucky Dreams, I was actually thinking of making her an Artificer and attack the party with steam-punk styled devices and items. Although they'll work like magic items (mostly, a few Rule of Cool handwaves will be employed), I think the look alone will throw off the players. They're veteran D&D players so anything that isn't a classical magic item has tended to catch them by surprise.
Tangent: My daughter was playing Pony Fantasy VI and mentioned that instead of Faust, Gogo should be replaced by Mare Do Well. That's not a bad bit of brilliance on her part...