A common plotline could be that Ash discovers Mallow has betrayed him in some form, perhaps by sharing a secret he entrusted to her. This then leads to a rift in their relationship and Ash has to decide whether to forgive her or not.
Another plotline could be that Ash unknowingly betrays Mallow first, maybe by accidentally getting her into trouble with a powerful group in their world. Mallow then retaliates in a way that Ash perceives as a betrayal, and they have to navigate through a whole mess of misunderstandings and hurt feelings to get their relationship back on track. This kind of plotline plays on the idea of miscommunication and the consequences of actions that may not have been intentional.
Another theme is self - discovery. Ash Mallow usually learns more about himself and his powers during the course of the fanfiction. For example, in some stories, he realizes his true potential when faced with great challenges.
One common theme is friendship. Since Ash and Mallow are friends in the Pokemon universe, many fanfictions explore the depth of their friendship, like how they support each other during tough battles or in their personal growth.
One interesting plotline could be Mallow and Ash going on a special cooking adventure where they combine their skills. Mallow's cooking knowledge and Ash's love for Pokémon could lead them to discover a new Pokémon that is attracted to Mallow's special dishes. They might then have to figure out how to catch it while also dealing with some challenges from Team Rocket who want to steal the Pokémon for themselves.
One popular plotline is Ash and Mallow going on a special adventure to discover a new type of Pokemon. They might face challenges like difficult terrains or rival trainers. Another could be a story where they have to save a group of Pokemon from a natural disaster. For example, a flood that endangers the habitats of some water - type Pokemon. And sometimes, there are fanfictions where they participate in a grand Pokemon tournament together, with their combined skills and strategies being the key to victory.
I'm not sure of specific 'ash and mallow betrayed fanfiction' titles off - hand. You could try searching on fanfiction - specific websites like Archive of Our Own or FanFiction.net. There are likely many stories out there that deal with themes of betrayal involving these characters, but you'll need to dig through the search results a bit.
Love and affection are common themes. Since there's a kiss involved, it often explores the budding or established relationship between Ash and Mallow. Another theme could be discovery, like the characters discovering new feelings for each other through the kiss.
Another theme is the development of their relationship. It often starts from being friends or comrades in the Pokemon journey and then slowly blossoms into something more romantic. Writers like to explore how they gradually realize their feelings for each other through different events, like helping each other out in tough battles or sharing a special moment during a festival in the Pokemon world.
One common theme could be friendship. Ash and Mallow might be shown having a deep and meaningful friendship that develops into something more. Another theme could be adventure, with them going on quests or exploring new places together.
One common plotline could be that Sabrina, perhaps feeling overlooked or underestimated by Ash, decides to betray him in a Pokémon battle. She might use underhanded tactics to win, which causes a rift between them.
A plotline might involve Ash being betrayed by a new acquaintance he meets on his journey. This person could pretend to be his friend, gain his trust, and then use that trust to steal his Pokémon or important items. Or perhaps Ash is betrayed by a Pokémon he just caught. The Pokémon could have been planted by an enemy trainer to spy on Ash and then turn on him at a crucial moment.