Maybe Team Rocket. They've been after Ash for a long time. If Ash comes back as a champion, they might see it as a threat and betray him in some way. They could pretend to be his friends at first and then stab him in the back to get what they want, like his powerful Pokémon or his championship title.
In this fanfiction, it may center around Ash's return with a new goal. For example, he could be aiming to uncover a mystery that has been hidden in the Pokemon world for ages. He might meet new allies along the way, and together they could explore uncharted areas or try to solve problems that are threatening the balance of the Pokemon world. There could be a lot of character development for Ash as he faces these new situations, and we might see him grow not only as a trainer but also as a person.
The theme of self - discovery is also quite common. Ash may have realized his true feelings for Serena while on his travels, and Serena might have also discovered more about herself in his absence. Additionally, the theme of friendship is often intertwined. Their past friendship forms the basis for their new relationship when Ash returns.
In many Pokemon fanfictions about Ash's Charizard's return, it often brings a sense of excitement and nostalgia. Usually, Charizard comes back stronger than ever, ready to help Ash in his new battles. It might have learned new moves during its time away, which makes it an even more formidable partner for Ash.
Some fanfictions might also include new characters that Ash meets on his return journey. These could be trainers with unique Pokemon or mysterious characters who have a connection to the events that led to Ash's return. For example, a young trainer who idolizes Ash and wants to learn from him, or a character who has information about a powerful Pokemon that Ash is interested in.
Love and reunion are very common themes. The idea of Ash coming back to Serena after a long time apart, and them picking up where they left off or starting a new chapter in their relationship.
It could be about Ash's return in a Pokémon story along with an original character (OC). Maybe it details new adventures they embark on, battles they face, and how the OC interacts with Ash and the Pokémon world.
Serna herself could be a main character. She might be Ash's love interest or a sister - like figure. Her reaction to Ash's return would be crucial in shaping the story. Maybe she is overjoyed at first but then realizes that Ash is not the same person who left for war.
There's a fanfiction called 'Ash's Homecoming to Serena'. In this story, Ash returns to Serena with a newfound sense of purpose. The story focuses on how their relationship evolves from friendship to something more romantic. It has some great character development for both Ash and Serena.