One common theme could be friendship. Despite the potential for romantic undertones, they might start as friends and support each other in their Pokémon - related adventures.
I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide variety of scenarios, but this particular one might be a bit unusual or not very common. You could try searching on major fanfiction websites to see if it exists.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ash Cynthia Diantha' on their search bars and you'll likely find a bunch of relevant fanfictions.
One great fanfiction could be 'The Journey of Ash with Cynthia and Diantha'. It has an interesting plot where Ash travels with the two strong trainers, Cynthia and Diantha, and they face various challenges together. They encounter powerful Pokémon and unique terrains. Another good one is 'Ash's Encounter with Cynthia and Diantha's Worlds'. In this, Ash gets to experience the different training styles of Cynthia and Diantha, which is really fascinating as he learns new techniques and strategies from them.
Finding good 'ash x cynthia x diantha fanfic' might take some time. You could start by looking at Pokémon - related fanfic communities. Sometimes, fans will recommend their favorite multi - character relationship fics in those groups. It's also a good idea to check the tags and summaries carefully to make sure it's the type of story you're interested in.
One common theme is the underdog's victory. Ash is often seen as the determined underdog compared to Cynthia's established strength, so his win represents the triumph of hard work and perseverance.
Writing such fanfiction that involves sexual content is inappropriate and goes against ethical and moral standards in most cases. Fanfiction should generally focus on positive, creative, and non - explicit relationships and stories.
Romance is also a big theme. Writers explore the potential romantic relationships between Ash, Cynthia, and the other characters in the harem. They often create unique love triangles or more complex relationship dynamics.
Romance is, of course, a major theme. It can range from the shy beginnings of their relationship to more passionate and deep emotional connections. Writers often explore how they fall in love and what draws them to each other. For example, Ash's determination and Cynthia's elegance might be aspects that attract them. Also, friendship is sometimes intertwined with the romance. Their existing friendship as fellow trainers can be the foundation for their romantic relationship in these fanfictions. And they might face challenges together as friends first before realizing their romantic feelings.
Popular themes often involve Pokémon training techniques. Ash, with his experience traveling through different regions, and Cynthia, known for her expertise, could exchange knowledge. Another theme can be cultural exchange. Since they come from different regions in the Pokémon universe, they could introduce each other to the unique Pokémon, traditions, and cultures of their respective homes. Also, a theme of redemption can be present, like if one of them makes a mistake and the other helps them to make up for it.