One common theme is Serena leaving Ash for another trainer. This could be because she is lured by the other trainer's success or power. Another theme might be Serena being forced to betray Ash due to some external threat or blackmail. For example, a villainous team might threaten her friends if she doesn't side with them against Ash.
Often, in these fanfictions, there's a theme of Serena feeling unappreciated by Ash. Maybe she's been with him through a lot, but he doesn't seem to notice her efforts or feelings. So, in a sense of self - preservation or seeking validation elsewhere, she betrays him. It could also be about Serena getting involved in a power struggle within the Pokemon world and choosing the wrong side which is seen as a betrayal to Ash and their friends.
One common theme is Serena getting jealous over Ash's interactions with other female characters. For example, if Ash is spending a lot of time with a new trainer girl. Another theme could be Serena's internal struggle with her jealousy, like trying to hide it from Ash. And sometimes, the jealousy can lead to misunderstandings between Serena and Ash, which then drives the plot of the fanfiction as they try to resolve their issues.
In such fanfictions, a prevalent theme is broken trust. Ash and Iris could have been on a journey together when suddenly Ash meets someone new who convinces him to change his path, leaving Iris behind. This would make Iris feel betrayed. Another theme could be related to secrets. If Ash has been hiding something important about his Pokemon training methods from Iris, and she finds out in a bad way, it can create a sense of betrayal. Also, the theme of jealousy can come up. If Ash gets more attention or praise from other trainers for his Pokemon, Iris might feel left out and betrayed, thinking Ash doesn't care about her feelings or achievements.
Love and relationships are also quite common. Whether it's her relationship with Ash in a new light because of her mermaid form, or new friendships she forms with other water - type Pokemon. In some fanfictions, there's a theme of acceptance. Serena has to deal with how others will accept her as a mermaid, especially those who are close to her like her friends and family in the Pokemon world.
One common theme is love and relationship development. It often shows how Serena and Ash's feelings for each other grow over time. Another theme could be adventure. The kiss might happen in the middle of an exciting Pokemon adventure. And of course, there's the theme of friendship evolving into something more.
A frequent theme is the support they give each other. Serena is always there to cheer Ash on in battles, and Ash helps Serena become more confident in her own abilities. Another theme is the idea of first love, as it's often portrayed as a pure and innocent feeling in these fanfictions.
Some fanfictions like to play with the idea of alternate universes. For example, what if Ash and Serena met in a completely different setting? Or what if their roles were reversed in some way? These alternate universe concepts can lead to really interesting and unique stories that still revolve around their relationship.
Adventure is a major theme as well. They are all on Pokemon - catching adventures, and fanfictions often take them on new and exciting quests. Maybe they discover a new and rare Pokemon species or explore an uncharted region. These adventures not only test their skills as trainers but also strengthen their bonds with each other.
One common theme could be jealousy. Maybe Serena is jealous of Ash's attention to his Pokemon or other friends. Another theme might be competition. She could hate Ash because she sees him as a rival in the Pokemon world. Also, it could be due to misunderstandings. Ash might do something that Serena misinterprets and that causes her to dislike him.
A lot of fanfictions also focus on Serena's growth as a character. For instance, how she becomes a stronger trainer or overcomes her insecurities with Ash's support. And sometimes, there are alternate universe stories where their relationship or their Pokemon journeys take a completely different turn.
Growth and self - discovery are also themes. Through the act of kissing Ash, Serena may be seen as taking a step forward in her own personal growth. She is being bold and expressing her true feelings, which can be a form of self - discovery for her character. For Ash, it might also be a moment of realizing his own emotional development and how Serena has become an important part of his life.