In this fanfiction, it could be that Ash's marriage is a way to bring peace between different regions. He marries a princess or a powerful trainer from a rival region. Their union symbolizes the unity of Pokémon trainers everywhere. The story could then detail the political and social implications of their marriage, as well as the more personal aspects like their relationship building and how they learn to love and respect each other's Pokémon.
Misty is a very likely candidate. She has been with Ash from the early days of his journey. They have a deep bond and have shared many adventures together. Their relationship has always had a bit of a romantic undertone, so in a fanfiction, it would make perfect sense for them to get married.
One possible plot could be that Ash and Ursula meet during one of Ash's travels. They start off as rivals but gradually develop feelings for each other. After many adventures together, they decide to get married. Their wedding could be attended by all their Pokémon friends, and they might start a new life exploring new regions as a married couple.
A plot could start with Latias being in some kind of danger, and Ash comes to her rescue. This leads to them spending more time together. As they get to know each other better, they realize their feelings are more than just friendship. After getting married, they might have to deal with people or Pokemon who don't approve of their relationship at first. But through their kindness and the strength of their love, they win them over and continue their happy life together.
One plot idea could be them training a new and rare Pokemon together as a married couple. Another could be them hosting a Pokemon tournament in their hometown.
The plot could involve Ash stumbling upon an Espeon that has lost its memory. Ash tries to help it regain its memories. Along the process, Espeon starts to remember bits and pieces which lead them to different locations. These places hold secrets and also some powerful Pokémon. Ash and Espeon team up with his other Pokémon to uncover the secrets and protect the area from villains.
Well, the plot could involve them reuniting after a long time apart. Maybe Ash has been traveling in different regions while Dawn has been working on her Pokemon contests back home. When they meet again, they discover that the spark is still there. Their shared love for Pokemon and the memories they've created over the years lead them to fall in love and get married. It could also include some challenges like dealing with some jealous rivals who try to break them up, but of course, their love prevails in the end.
In the fanfiction, Ash and May could get married in a secret location, away from the public eye. They've had a long - standing relationship that they've kept private. The story could revolve around their journey to this secret location, with only their most trusted Pokemon by their side. Along the way, they reminisce about all their adventures together and how they fell in love. Once they reach the location, it's a simple yet intimate wedding with just a few close friends and their Pokemon.
In this fanfiction, Ash might take really good care of the Pokemon egg. He would probably keep it warm and safe, waiting eagerly for it to hatch. Maybe he'd even talk to the egg, like he does with his other Pokemon friends.
The storyline might involve Jessie leaving Team Rocket to start a new life. She could meet someone who is not into the whole villainy thing. Their relationship could develop as they both discover a shared love for Pokemon in a different way than she did with Team Rocket. For the wedding, it could be in a beautiful meadow filled with wild Pokemon. Some of the Pokemon might even sing or create a beautiful display to celebrate the union.
In this fanfiction, if Ash turns into a pokemon, perhaps he becomes a Charmander. He could struggle with being in a different form. He might have to adapt to the life of a fire - type pokemon, facing challenges like being weak against water - type attacks and trying to find a way to change back while still having adventures with his friends, who now have to protect him in a different way.