I've always had a story idea were Sunset was a bad girl rebel character and Twilight was the evil preppy popular girl of CHS with the Mane Six as her posse. I tried writing it but it never got far.

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That actually is an interesting concept, the best way I can think of it going would be Human Twilight being just as smart, but being kicked out of crystal prep for being a bully and such. So she gets sent to Canterlot instead, and things go on. Only thing I can think of is you'd have to adjust Sunset's background a bit to abide by her character change, same thing for Twilight. Would Pinkie and the others also be evil?

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Well my idea conceptually would be Twilight would be like a smart know it all stuck up rich girl with a lot of brain power.

Sunset would be more rocker biker bad girl type.

Pinkie and the others......IDK....You could alter them to be coerced by social pressure into being bad (Like one fic I read a while back) or they could be bystanders.

Honestly that would be for the author to decide.