A fanfiction could focus on how the ponies form an unlikely alliance with some of the Storm King's minions. They realize that they also don't like the Storm King's tyrannical ways and decide to work together to overthrow him. This would involve a lot of character development for both the ponies and the minions.
One idea could be that Twilight discovers a way to redeem King Sombra. She uses her magic and knowledge to break through his dark exterior and find the good within him. Maybe she uncovers a tragic past that led him to become so evil.
One plot idea could be that Twilight has to enter the realm where King Sombra was banished to retrieve a powerful magic artifact. Along the way, she discovers secrets about Sombra's past that make her question if he was truly all evil. Another could be a time - travel plot where Twilight goes back in time and has to prevent King Sombra from ever rising to power in the first place. Maybe she teams up with an ancestor of one of her friends.
One idea could be a pony being reborn with memories of a past life in a different pony world. They could use that knowledge to bring about positive changes in Equestria, like finding new ways to solve friendship problems.
Well, a plot could involve a villain trying to steal all the cupcake recipes in Ponyville. The ponies then have to work together to protect their precious recipes. It could also be about a pony who has lost their sense of taste and has to go on a journey to regain it by trying different cupcakes. Or perhaps there's a mystery surrounding an ancient cupcake recipe that holds a great secret.
One idea could be that the Dazzlings try to reform and join the main group of ponies in Ponyville. They struggle to overcome their past of using dark magic to steal harmony and learn to use their powers for good.
The Storm King himself is very interesting. He has a menacing presence and his motives for conquering are open to exploration. You could delve into his past to find out what made him so power - hungry.
Perhaps they could form an unlikely friendship. Rumble, being a pegasus colt, and Sweetballe, with her own unique charm, might start off not getting along but then discover they have more in common as they work together to help a group of young ponies learn about flying and friendship. They could then have to defend their newfound friendship from those who don't understand it.
One idea could be a time - travel plot where the Equestria Girls go back in time to the founding of Canterlot High. They could witness the first magic being used there and have to ensure that history isn't changed too much. Another idea is a crossover with another magical school from a different franchise. For example, they meet students from Hogwarts and have to learn from each other's magic systems. And a third idea could be a story where they lose their magic and have to go on a quest to find a way to get it back, encountering many challenges and new friends along the way.
One idea could be a time - travel plot where a character from Canterlot High goes back in time to the founding of the school and has to make sure certain traditions are established correctly. Another could be a story about a new student at Canterlot High who has a special power and has to learn to control it while fitting in with the existing groups, like the Rainbooms or the Shadowbolts. A third idea is a mystery - solving plot where something precious is stolen from the school and the students have to work together to find it.
One idea could be a female OC who is a talented artist in Equestria but is too shy to show her work. She then meets one of the main ponies like Twilight Sparkle who encourages her to share her art with the world. Another could be a female OC from a different land who comes to Equestria to learn about friendship and has to overcome cultural differences.