When you think about it, it would've been more fitting to have the Main Six's evil opposites be the antagonists of the final season. It definitely makes sense. They would have to experience the same teamwork lessons the Legion went through, though probably not have the same plan. My vision would be that they craft their own Elements of Disharmony, which could not only cause untold destruction when together, but were capable of tainting ponies with their vices while separate. A brilliant idea would be that they use their appearances to frame the Main Six and tarnish their reputations while also causing the pony tribes to turn against each other to the point that the Wendigos return. Finally, on the day of Twilight's coronation, Fake Twilight's plan could be to destroy Twilight and take her place to become ruler of Equestria, promising to bring peace back to the land and get rid of the Wendigos (which of course would be a bunch of hogwash). Even more, they could use Fake Fluttershy to gain control of monsters from the Everfree Forest and Tartarus to form an army.

All in all, this could've been a great plotline for the final season, and it seems like a missed opportunity.

Love it xD