If you were to create the last enemy of the show, who would it be? It can be anyone except for the canon villains.

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Can it be a G1 villain?

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I am going to assume you meant villains that were already featured in FiM and not the entire MLP franchise as a whole? The creators of the show showed they could take older designs and translate them well, into the current MLP style, like with Grogar and Tirek. If I could pick two older vilains, I would choose the Real Grogar or Aramus.

Grogar because he is iconic and could have been implemented really well as a villain. Heck you could have brought back Tambleon as this transylvannia-like city state that exists far away from Equestria where he was slowly rebuilding his strength and is still looking to get his bell back. And then reclaim Equestria as his own.

Aramus would work well as an enemy, his whole theme is shadows. Power over and stealing Shadows, he could be tied back into the Pony of Shadows, potentially responsible for the creation of this villain. Maybe he's the one who first taught Sombra shadow magic, which lead him to becoming obsessed with Dark Crystal magic? Maybe if you wanna take it a step further, he's partly responsible for turning Princess Luna into Nightmare Moon. His whole thing has been spreading fear and mistrust from the shadows.

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How about that female, G1 villain who would change the reformed villains into various objects like bubbles, trees, and stone due to thinking they didn't deserve redemption for their actions? Sorry. I forgot her name, but it starts with an M.