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 a crossover with another popular animated series like Steven Universe. The characters from Equestria Girls could visit Beach City and interact with the Crystal Gems. For example, Twilight Sparkle might be intrigued by Garnet's future vision ability and they could have a discussion about the nature of time and knowledge.
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.
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 that your self - insert character is a new student at Canterlot High. They have a special talent for art but are really shy. As the story progresses, they get involved with the main characters like Twilight Sparkle and her friends. They help with a school art project that turns out to be a magical adventure. Another idea is that your character is a transfer student from a different dimension entirely and has unique magical powers that the Equestria Girls haven't seen before. This causes both excitement and some initial suspicion among the group.
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.
Another plot idea is to create an alternate timeline within the 'a walking shadow' world. This could involve different events happening to the characters, leading to new relationships and challenges. For example, a character who was once an enemy could become an ally in this new timeline.
For Shadow of the Colossus fanfiction, you could write about a hidden society within the Forbidden Land. They worship the colossi in a different way than the main narrative implies. Wander stumbles upon this society during his journey. Or, you could do a fanfic where the colossi are not actually the 'bad guys'. They are guardians of something far more important and Wander is being used by the shadowy forces that sent him on this quest. And finally, a story about how the magic or energy that powers the colossi affects the wildlife in the Forbidden Land in ways not shown in the game. For instance, there could be mutated or super - intelligent animals due to this energy.
One idea could be a character discovering a hidden power within the shadows that they must learn to control. Maybe they are initially afraid of it but then realize it can be used for good.