I was thinking, not for s6, maybe not even for gen4, but maybe for gen5 or something.
What if Flurry Heart became evil? It could be almost like this-
Sombra comes back. (Remember, his red horn stayed in one piece when he got destroyed. Perhaps using the dark magic in his horn, he came back? Kinda like Sauron and The Ring in LOTR) He finds out about Flurry, and decides to use her. He then proceeds to start meeting up with her, and teaching her dark magic. Maybe she falls in love with him.
Then, once she is grown up, she revolts against her parents and Equestria, Sombra at her side. Her crazy magic is too much for the other Princesses. Then Twily and her friends save the day -_-
Maybe banishing her and Sombra? Idk.
^now that I look at it, it seems to be more like an epic fanfic O__O feel free to use it just tell me first please :3)