A less common but still interesting theme is the exploration of their backstories. Writers might dig into Magnus's past, how he became who he is, and how Cleo fits into that story. Or they might explore Cleo's family and how Magnus impacts her relationship with her family. This can add a lot of depth to the characters and their relationship in the fanfiction.
Romance is also a big theme. Fans love to explore the relationship between Magnus and Cleo, from the first sparks of attraction to a deep, committed love. They might write about their first date, how they confess their feelings, or how they deal with relationship problems. Another theme could be magic. Given that they might be characters in a magical world, fanfiction could explore how they use their magical abilities together, whether it's to save their world or just to have some fun.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfiction from various fandoms, including Magnus and Cleo. Just use the search function on the site to look for it.
Adventure - based themes are also quite popular. Writers might create scenarios where Cleo and Deuce go on an exciting adventure together, facing challenges and growing closer as a result. It could be something like exploring a mysterious new world or solving a spooky mystery at their school.
Another common theme is character relationships. This could be romantic relationships, like a pairing between a Magnus Chase character and a Percy Jackson character. Or it could be friendships, exploring how characters from the two different series would be friends.
Another theme is the growth of their relationship. In married fanfiction, it's often about how they become better versions of themselves because of their partner. Like Deuce becoming more confident with Cleo's support.
One popular theme is the first kiss. Writers often explore the nervousness and anticipation of Alec and Magnus before their first kiss. It could be set in a romantic location like on a rooftop under the stars.
One popular theme is friendship. Writers might explore how Magnus and Alex become closer friends, sharing secrets and having each other's backs. Another could be adventure, where they go on quests together and rely on each other's skills.
One of the most popular themes is the exploration of their cultural differences. Magnus being a warlock with a long and rich history and Alec being a Shadowhunter from a more traditional background. Another theme is their struggle against prejudice. They face discrimination from the Clave and others due to their relationship. Also, the theme of sacrifice is common, like how they would sacrifice for each other's safety and happiness.
Emotional hurt is also quite prevalent. For example, his relationships with the Shadowhunters might cause him pain. Maybe a betrayal by someone he trusted, like Alec or one of the other characters, which can lead to a lot of angst in the fanfiction.
One common theme could be the relationship between Hercules and the Muses, especially with Cleo Lemon in the mix. It might explore how their interactions develop over time.
One common theme is isolation. Magnus might be isolated due to his unique powers or past traumas, which causes him pain. For example, in some fics, his immortality makes him feel alone.