One common theme could be love and affection. Since it involves a 'beso' (kiss), the fanfic may focus on the deepening relationship between Ash and Serena.
There are many fanfic platforms out there. Some popular ones apart from Archive of Our Own are FanFiction.net. Browse through their Pokémon section and look for stories with Ash and Serena in the tags. Then, you can skim through the descriptions or reviews to find the ones that might have a 'beso' scene as you desire.
One idea could be Serena and Ash getting lost in a mysterious forest in the Kalos region. They have to rely on each other's skills and the Pokemon they have with them to find their way out. Along the way, they might discover a new and rare Pokemon species.
Their relationship could develop further through shared adventures. For example, if they team up to rescue a group of endangered Pokemon, they would have to rely on each other's strengths and trust each other completely. This would deepen their bond.
There could be. The Pokemon fandom is quite large and diverse. Fanfic writers love to create unique storylines for their favorite characters. Ash and Serena are popular characters, so it's not surprising that someone might write a fanfic about them having a family. But these types of fanfics may not be endorsed by the official Pokemon creators.
One great fanfic could be 'The Journey of Ash and Serena' which focuses on their adventures together. It has a lot of sweet moments between them and shows how they grow both as trainers and as friends.
A possible fanfiction plot could be that Ash, Serena, and May are invited to a mysterious island full of unknown Pokemon. They have to work together to survive and discover the secrets of the island. Ash's leadership, Serena's ability to communicate with Pokemon, and May's quick decision - making skills would be crucial.
One reason could be that she might be worried about Ash getting into dangerous battles. She cares deeply for him and the thought of him getting hurt scares her.
Definitely their close friends like Brock and Misty. They would be there to support Ash. And all of Serena's friends from Kalos too. Also, the Gym Leaders they've met along their journey might be invited. And of course, their Pokemon. Pikachu would be right there by Ash's side, and Serena's Fennekin would also be present.
Well, it could start with Ash finally realizing his true feelings for Serena. Maybe they meet up again after a long time apart. Then they start to plan a small, intimate wedding in a beautiful Pokemon - filled location like a meadow full of Pikachu and Eevee. Their Pokemon friends could help with the preparations, like Pidgey delivering invitations.