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.
Growth and self - discovery also play a big role. Both Ash and Serena are on their own personal growth journeys. In the fanfictions, their relationship can be a catalyst for their individual growth. Serena might become more confident in her skills as a Pokémon trainer because of Ash's support, and Ash might learn more about his emotions and how to handle them through his relationship with Serena.
Another great one is 'Ash's Discovery of Love'. It focuses on Ash's side of the story. How he slowly realizes his feelings for Serena during their travels. It has some really cute scenes where he starts to see Serena in a different light, and their relationship blossoms from there. The story also includes their Pokémon partners, adding an extra layer of charm to the narrative.
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.
They would probably be a bit shy at first. Ash being his usual energetic self but also respectful of Serena's space in the tent. Serena might be a bit nervous but also excited to be sharing such a close space with Ash.
In fanfiction, anything can happen. However, without specific details of the particular fanfic, it's hard to say for sure if Serena is pregnant. Some fanfictions might explore this plotline for the sake of drama or character development. It could add an interesting twist to the relationship between Ash, Serena and Lemon, but it also depends on the overall theme and direction the author wants to take the story.
A great idea could be a fanfiction about Ash and Serena traveling together to a new region to discover rare Pokémon. They could face challenges, make new friends, and help each other grow as trainers. This would focus on their relationship in the context of the Pokémon world and their shared goals.
For Ash, he could develop better communication skills. In the Kalos League, he often gets so focused on battles that he forgets to share his thoughts and feelings with Serena. By learning to communicate better, he can strengthen their relationship. For Serena, she could develop more self - confidence in her own Pokémon training skills. Maybe she starts out doubting herself but as the Kalos League progresses, she becomes more assertive and makes her own mark in the Pokémon training world.