Another thing that might occur is that Ash brings back some sort of danger or enemy from the war. This could put Serana in jeopardy and they'd have to face it together. It would be a test of their relationship and their individual strengths. They might have to rely on each other more than ever before.
Well, one possibility is that Ash has changed a great deal due to the war. Maybe he has new skills or a more hardened personality. He could struggle to fit back into his old life with Serana. For example, he might be haunted by the memories of the war and find it difficult to be as carefree as he once was with her.
She could be overjoyed at first. After all, she might have been worried sick about him during the war. However, she may also be a bit hesitant. The war could have changed Ash in ways she doesn't understand yet.
Well, perhaps Ash comes back with new skills and a sense of purpose. He might be determined to protect Serena even more than before. In the fanfiction, he could use his war - learned strategies to deal with any threats that come their way. And their relationship could evolve into a deeper, more mature love as they face new challenges together.
Perhaps he'll share his new adventures and the knowledge he's gained from traveling different regions. He could inspire the new generation of Pokémon trainers in Pallet Town to aim for greater things.
In this fanfiction, Greninja might come back with new powers. Maybe it has been on a special training journey and returns to Ash to help him in a big battle.
In this fanfiction, Ash might introduce Greninja to his family. His mother could be surprised but also happy to see such a unique Pokémon. Maybe Greninja would show off some of its cool moves in the backyard.
In the fanfiction, they may decide to explore uncharted regions. With their combined knowledge and skills, they would be a formidable team. Along the way, they could discover new Pokémon species and form alliances with local Pokémon trainers. Their marriage would be strengthened by these shared adventures, and they would become legends in the Pokémon world for their discoveries and contributions.
Latias could initially go into hiding, feeling hurt and betrayed. She might avoid Ash at all costs. Then, Ash could realize his mistake and start searching for her to apologize.
The first thing that could happen is that his friends would immediately start to tend to his wounds. Erza, being the strong and reliable one, might take charge of the situation. She could use her medical knowledge, if she has any in the fanfiction world, or she could call for help from a more experienced healer. This would show the bond between Natsu and his friends.
Well, in this kind of fanfiction, Obito might face a lot of challenges in readjusting to the post - war world. Maybe he has to deal with the guilt of his past actions.
His best friend from before the war could be a main character. This friend might have been waiting for Ash's return and now has to deal with the changed Ash.