A common theme is adventure. Since Simba has such an eventful life, the reader often gets involved in his adventures, like fighting against Scar or exploring new parts of the Pride Lands. Another theme is love. It's not uncommon to see a romantic relationship develop between Simba and the reader, which can be a bit of a forbidden love since they are from different worlds in a way.
One more theme that pops up a lot is self - discovery. The reader, through their interactions with Simba, discovers more about themselves. For example, they might learn to be more brave like Simba. Also, the theme of friendship is there. Even if it doesn't turn into a romantic relationship, the bond between Simba and the reader can be a strong friendship that helps them both through tough times. This is especially important considering Simba's own journey of dealing with loss and reclaiming his place.
One common theme is friendship. Despite their differences in the original story, fanfictions often explore how they can become friends. Another is leadership, as both are important lions in the 'Lion King' world. They might learn from each other about leading the pride.
One common theme could be redemption. Kovu has a complex past and his relationship with Simba can be a vehicle for exploring how he can redeem himself in Simba's eyes or in the eyes of the pride.
The theme of unlikely alliances also pops up a lot. Given their history of enmity, the idea of Scar and Simba forming an alliance, whether it's to fight a common enemy or for the good of the Pride Lands, makes for a very interesting narrative in many fanfics.
The theme of family is also quite common. They have cubs in the official story, and fanfictions might expand on how they raise their family. Also, adventure is a theme. They might go on adventures to protect the Pride Lands or to discover new places, which is often depicted in these fanfictions.
Another theme is self - discovery. The Oc may help Simba learn more about himself, his leadership skills, or his place in the circle of life. And Simba in turn may help the Oc understand the ways of the Pride Lands and the importance of community.
A sense of belonging is another big theme. The reader, who may feel like an outsider in the real world, finds a place where they belong among the mutants. There are also themes related to fighting for justice, just like the X - Men do in the original stories.
One common theme is adventure. Since Chekov is part of the Star Trek universe, many fanfictions involve the reader going on adventures with him through space, exploring new planets and facing various challenges.
Love and sacrifice are also very common. The reader might fall in love with Vergil and be willing to sacrifice for him, or vice versa. In some fanfictions, there is also the theme of discovery. The reader discovers new sides of Vergil that no one else has seen, which deepens their relationship.
One common theme is adventure. Since boboiboy is often involved in battles and saving the world, the reader character gets to be part of those exciting adventures in the fanfiction.
One common theme is growth. Both Armin and the reader often grow as individuals while their relationship also evolves. For example, they might learn to be braver in the face of danger.