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.
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.
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.
The theme of destiny often plays a role. Many fanfics portray Serena and Darien as being fated to be together. Their relationship is seen as something that was meant to be from the start. This gives an air of inevitability to their love story and adds an element of magic and wonder. There are also themes of redemption in some fanfics, where one of them has to atone for past mistakes and their relationship is part of that journey.
One great Ash and Serena fanfic is 'A Journey Together'. It beautifully captures their relationship growth as they travel through the Pokémon world. The author does an amazing job of showing Serena's determination to support Ash in his Pokémon battles and how Ash starts to see Serena in a different light.
Serena could be a bit shy at first. She might blush deeply and look down for a moment. But then, she would look into Ash's eyes with love and admiration. She would tell Ash how much she loves him and how she has been hoping for this moment. She would accept his proposal gracefully, and they would share a tender embrace while their Pokémon friends cheer around them.