I think Professor Sycamore would be an interesting character to pair with Calem in fanfiction. Calem could be his assistant for a special research project on Mega Evolution. This would give Calem a chance to learn more about the secrets of Pokemon from the professor. Also, Tierno could be paired with Calem. They could have a dance - off with their Pokemon, which would be a really fun and unique scenario in the fanfiction.
A fanfiction could pair Calem with Diantha, the champion of Kalos. Calem could be mentored by her, and she could teach him some advanced battle techniques. Or perhaps they could team up to defend the Kalos region from a group of Team Flare remnants. This would create an exciting story with a lot of action and drama, as Calem would be learning from one of the best trainers in the region.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Pokemon X and Y fanfictions featuring Calem and Serena. Just use the search function and type in the relevant keywords.
One great fanfiction is 'A Journey with Serena and Calem'. It focuses on their adventures in the Kalos region, exploring how their relationship develops during battles and while facing various challenges. The author does a wonderful job of capturing their personalities as seen in the game.
One idea could be Calem's journey after becoming the champion. Maybe he travels to other regions to learn new battle strategies and meet different Pokemon. Another could be a story where Calem has to team up with his old rivals to face a new, powerful threat to the Kalos region.
The plot also matters a great deal. It could be something related to their Pokemon journey, like exploring new areas in Kalos that weren't fully covered in the game. Or it could be a more personal story, like Serena dealing with homesickness and Calem helping her through it. A good balance between action - packed Pokemon battles and emotional character moments is key. In a story I read, they were on a quest to find a legendary Pokemon, and along the way, they had heart - to - heart conversations that deepened their relationship.
One possibility could be that Calem's immortality is a result of some ancient Pokemon power. Maybe he stumbles upon a hidden temple in the Kalos region where a powerful Pokemon bestows this gift upon him. This would lead to him having to deal with the loneliness that comes with immortality as he watches his friends and family age and pass away.
The characters Calem and Serena themselves are very interesting. Their relationship in the game can be explored further in fanfiction, like their adventures together that might not be fully shown in the game. It gives fans a chance to see more of their dynamic.
His relationships with his friends would likely become strained. They would age while he stays the same. For example, his best friend might start to feel a sense of envy or resentment as Calem doesn't have to face the normal passage of time. His family might be confused and worried about his immortality at first.
Blue could also be an interesting character to pair with Leaf. Their different personalities would create a lot of interesting dynamics. Blue's cocky attitude could contrast with Leaf's more calm and collected nature, leading to some fun interactions and perhaps even some friendly rivalries.
Serena would do well with a Delphox, the evolved form of Fennekin. It has strong fire - based attacks. Additionally, an Altaria would be a wonderful addition. It's a flying - type with a graceful appearance. For Calem, Greninja would be a superb choice. It's extremely agile and has great water - type attacks. And a Tyranitar could be in his team too, as it's a very tough and powerful Pokemon.
Lillie is an obvious choice. In the original Pokemon story, she has a strong connection with Nebby. Their relationship can be further explored in fanfiction, like Lillie teaching Nebby new skills or Nebby protecting Lillie from danger.