One idea could be to explore a Changeling OC's struggle to fit into human society while hiding their true nature. For example, they might have to constantly lie about their abilities and origins, which could lead to some tense and dramatic situations.
Maybe a changeling OC who gets lost in Equestria during a mission and is found by one of the main ponies. The OC could initially be distrusted, but as the story progresses, they form an unlikely friendship. The OC could then start to learn about friendship and love from the ponies, which would change their view of the world and themselves. For example, they could start to see that there are better ways to get the energy they need than stealing love.
One idea could be exploring the origin of the changelings in more depth. Maybe there was a powerful magic that created them long ago.
One idea could be exploring how the changelings adapt to a new world order. Maybe they start to embrace new values and relationships that were previously foreign to them.
A Changeling could be sent on a secret mission to infiltrate Fairy Tail by an evil organization. But as they spend time with the guild, they start to have second thoughts and eventually switch sides, becoming a valuable member of Fairy Tail and using their shape - shifting abilities to help the guild in battles.
There might be a fanfiction where a changeling OC discovers a new power within itself. For example, it could have the ability to transform into more than just ponies, like other magical creatures in the MLP universe. This new power could lead to all sorts of adventures and self - discovery as it tries to figure out where it fits in the world.
Another plot idea might be a conflict between the traditional wizarding values and the more fluid and dream - like nature of the changelings. A changeling student at Hogwarts might face discrimination or misunderstanding from other students and teachers who don't understand their abilities. This could lead to a story of the changeling trying to prove their worth and show that their different nature can be an asset to the wizarding community.
One plot idea could be a changeling trying to fit into pony society without revealing its true identity at first. It could develop real friendships with ponies and then struggle with the guilt of deception. For example, it might fall in love with a pony and worry about how the pony will react when it discovers the truth.
Another plot idea might be that the wizarding world discovers Harry's changeling status and there is a big debate about whether he should be allowed to continue at Hogwarts. Some might fear his different nature, while others see it as an opportunity to learn more about the magical world's secrets.
One idea could be creating an OC who has a unique power related to time manipulation. They could be from a different dimension and end up in the Avengers' world. Another idea is an OC who is a master of ancient magic and teams up with Doctor Strange within the Avengers group. Maybe an OC that is a super - intelligent android created by a villain but has a change of heart and joins the Avengers.
You could create an OC for your Persona OC fanfiction who is an artist. This artist OC could use their art as a way to understand and even manipulate the Metaverse. For example, they might draw scenes from the Metaverse that give clues about upcoming events or hidden locations. Their art could also be a form of therapy for other characters who are struggling with their dual lives in the real world and the Metaverse.