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.
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.
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.
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.
One interesting plot could be the human adjusting to the magical world of Equestria. For example, learning to use magic or getting used to the different species and their cultures. Another idea is the ponies trying to understand human behavior and needs. They might be confused by things like human technology or the lack of a horn or wings.
One interesting plot idea could be a mystery during the wedding. For example, a valuable item goes missing right before the big moment. Another could be a character from the guests' past shows up unexpectedly and has some unresolved issues with the bride or groom. A third idea might be that there are some magical mix - ups that cause comical chaos during the ceremony.
A really interesting plot could be a pony from the other side who has unique powers that are considered dangerous in their own world. They flee to the normal pony world and have to hide their powers while also trying to find a place where they belong. This could involve a lot of character development as they interact with the normal ponies and learn about a different way of life.
Perhaps a story where Scootaloo and Rumble get into an argument over which pony tribe is better, the pegasi or the earth ponies. But then they are forced to work together when a strange magic starts affecting the ponies of their respective tribes. As they work together, they realize that each tribe has its own strengths and that they were wrong to argue in the first place.