Friendship is a big one. Just like in the original My Little Pony series, the idea of different characters coming together and forming bonds is common. Sonic, being from a different universe, has to learn to work with the Equestria Girls.
Sure. 'Sonic Meets Equestria Girls' is a popular one. It has an interesting plot where Sonic accidentally enters the world of Equestria Girls and has to find his way back while making new friends.
Another theme is the clash and then the melding of different cultures. Sonic's world has a more technological and action - based culture, while Equestria Girls has a world full of magic and school - based social structures. Fanfictions often play with how these two different cultures interact. Characters might be initially confused or in conflict due to the differences, but eventually they learn to appreciate and work with each other.
One common theme is friendship. Just like in the original MLP series, the Equestria Girls fanfiction often emphasizes the importance of friendship among the characters. They face challenges together and support each other through difficult times.
One great trope is the 'fish out of water' scenario. The main character from Equestria suddenly finding themselves in a human world and building a harem can lead to some really interesting and comical situations. For example, they might misunderstand human customs which then endears them to the other characters in the harem.
A really popular one is the fanfiction that focuses on Twilight's relationship with her friends in the Equestria Girls setting. It shows how they support each other through school problems, boy - girl drama, and still manage to keep their magical bond intact.
One common storyline is Sonic getting transported to the Equestria Girls' world and having to adapt to the new environment. He might have to deal with the different magic and social norms there. Another could be Sonic teaming up with the Equestria Girls to fight a common enemy, like a powerful villain from either his world or theirs. And sometimes, there are stories about Sonic teaching the Equestria Girls about his speed and abilities, and in return, they teach him about friendship in their unique way which is very different from what he experiences in his own world.
One interesting story could be about Sonic somehow getting transported to the Equestria Girls' world and having to team up with the girls to stop a new villain. Maybe he uses his speed to outwit the bad guy while the girls use their magic and friendship powers.
One popular sonic equestria girls fanfiction might be those that involve crossovers between Sonic characters and the Equestria Girls universe in a friendship - based adventure. For example, stories where Sonic and Rainbow Dash team up to save both their worlds from a common threat.
Don't rush the relationships. As I mentioned before, a slow - burn can be really effective. Let the characters get to know each other gradually. Also, add some conflict. It could be jealousy among the harem members, or an outside force trying to break them up. This adds drama and keeps the story interesting.