An OC who is a shy artist could be a great idea. She discovers a hidden art gallery in Canterlot High that holds some ancient magic. As she tries to explore it further, she draws the attention of the villains in the Equestria Girls universe. Her new friends from school then have to help her protect the gallery and its secrets.
One idea could be an OC who is a new student at Canterlot High. She has a unique power related to music, like she can make people feel different emotions through her singing. And she gets involved in a battle of the bands where there's some sort of mystery or magic influencing the competition.
Maybe there's a villain from the MLP world that follows them into the Equestria Girls world. The girls then have to team up with their Equestria Girls counterparts to defeat this villain. They could combine their different skills, like the pony Rainbow Dash's speed in the air and the Equestria Girls Rainbow Dash's skills on the track team.
One idea could be that Sonic accidentally enters the world of Equestria Girls while chasing Dr. Eggman through a strange portal. He then teams up with the girls to stop Eggman from stealing the magic that powers their world. Another could be a race between Sonic and Rainbow Dash which gets interrupted by a common enemy.
Well, an interesting plot could involve an OC who discovers she has a hidden talent for magic in the human world of Equestria Girls. She then has to hide this from her non - magical friends while also trying to figure out why she has this power. Maybe she teams up with the main characters to face a new threat that is using dark magic. She could also have a special connection to an item from Equestria that holds the key to defeating the threat.
One interesting plot point could be the culture shock the Equestria Girls experience when they first arrive in Equestria. They might be amazed by the magic that is so commonplace there but strange to them. Another could be their interactions with the ponies. For example, they might have to learn new ways of communication and social norms.
One idea could be a crossover where Shadow meets the Equestria Girls during a magical event in their world. Maybe he has to team up with them to defeat a common enemy that threatens both his world and theirs.
One idea could be an OC who has a unique power like the ability to control plants. She could team up with the Powerpuff Girls to stop Mojo Jojo from creating a giant plant monster to take over Townsville. Another plot could involve an OC from another dimension who comes to Townsville seeking help because her own world is being threatened by a similar evil as in Townsville. The OC could bring new powers and a different perspective to the fight against the villains.
One idea could be a human with unique skills, like being a great artist, arriving in Equestria. The ponies are fascinated by the human's different art style and the human has to learn about Equestrian art in return. For example, the human could introduce the concept of abstract art to the ponies, while learning about the magical paintings in Equestria.
One possible plot idea could be Twilight Sparkle discovers a new magic power in the Equestria Girls world that is related to her futa form. This power could help her solve a mystery or overcome an enemy. Another idea is that she has to deal with the challenges of being different in this form while still trying to fit in with her friends. Maybe she could use her unique abilities to help her friends in unexpected ways, like using her newfound strength to protect them from a bully or a dangerous situation.
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.
A cool fanfiction concept could be a crossover tournament. Ben 10 brings some of his alien allies, and the Equestria Girls bring their friends from Canterlot High. They compete in a series of challenges that test both strength and magic. Each team has to learn from the other's abilities and work together at times. This could lead to some interesting friendships and rivalries between the characters from the two different universes.