Before you read, this is about an OC called Moonlight Mirror. The action happens between the 8th and 9th season. That means that... (season 8 spoilers)
Cozy Glow is locked up, the school of friendship is running under Twilight's rule with the mane 6 being teachers.
But... (season 9 spoilers)
Twilight hasn't even thought about ever ruling equestria and the 2 princesses are still the proud rulers of Equestria.

Moonlight Mirror introduces herself as a villain, but she soon has to learn how to live amongst good ponies. She loves schemes and she seeks revenge on everyone who has tried to put her in her place.