Jessie and James from Team Rocket could be fun to include. They could initially try to disrupt the wedding as part of their Team Rocket antics, but then end up being touched by the love between Ash and May and decide to give up their evil ways for a day to celebrate with them. Also, Dawn, one of Ash's later friends, could be there. She could help May with the wedding preparations and share some stories about Ash with her. And Gary, Ash's rival, could be at the wedding too. He could give Ash a friendly jab during his best man speech.
Obviously, Brock should be included. He has been Ash's friend for a long time and could be Ash's best man. He would also add some comical moments to the story, like when he flirts with the female guests at the wedding. Max, May's little brother, would also be a great addition. He could be a bit of a troublemaker at first, not fully understanding why his sister is getting married, but then come around and be really happy for her. And of course, all of Ash's Pokémon, like Pikachu, should be there. Pikachu could even be the ring - bearer in a really cute way.
Definitely their Pokémon friends. Pikachu, of course, would be there. May's Blaziken and Ash's Greninja would also be among the guests. Their human friends like Brock, Misty, and Dawn would surely come. And perhaps some of the Gym Leaders they've met during their travels would also attend, like Roxanne from Rustboro City.
Definitely Toad. Toad has been a loyal friend to both Mario and Peach. He could be the one helping with all the little details of the wedding, like arranging the flowers or setting up the chairs. Yoshi could also be a great addition. He could carry Mario and Peach on their honeymoon as a fun way of transportation. Luigi should be there too, as Mario's brother, he would be an important part of the celebration.
Quinn would be an important character to include. Given her past relationship with Puck, her presence at the wedding could add some drama and tension. Santana could also be part of it. She could be the sassy friend who makes sarcastic comments about the wedding but deep down is happy for them. And of course, Mr. Schue. He could give a heartfelt speech at their wedding about how far they've come.
The other demigods from Camp Half - Blood should be included. People like Clarisse, who at first might seem grumpy about the whole wedding thing but then has a change of heart and gives a really nice gift. And Nico, who could use his powers to create some amazing lighting effects during the evening reception. Their presence would add a lot of flavor to the story.
In an 'Ash and May get married' fanfic, perhaps they reunite after a long time apart. May has become a renowned Pokémon coordinator, and Ash is still on his journey to be a Pokémon master. When they meet again, the old feelings resurface stronger than ever. They face some challenges from their families who have different expectations for them, but they overcome these obstacles and get married. Their wedding could be held in a big city with a Pokémon parade through the streets.
I think Serena should be in an Ash meets Dawn fanfic. She has a special connection with Ash, and her presence could add an element of jealousy or confusion in the relationship between Ash and Dawn. Also, Cilan. His expertise in Pokemon types and his positive attitude could help Ash and Dawn in their adventures. And finally, Gary. His rivalry with Ash could be reignited in the context of the story, and Dawn could be caught in the middle of it all.
Definitely their Pokémon. Pikachu has been with Ash since the beginning and it would be great to see how it interacts with the family. Serena's Fennekin (which evolves later) can also be an important part of the family, adding a cute and fiery element.
Definitely Team Rocket. Their constant attempts to steal Pokemon would create a lot of action and comedic moments. Nurse Joy is also important as she can heal their Pokemon when they are injured. Officer Jenny can be there to enforce the law when Team Rocket causes trouble. And Cilan from the Unova region. His expertise in Pokemon types and flavors could add a unique aspect to the story.
Gary Oak could be an interesting addition. His rivalry with Ash is well - known, and his skills as a Pokemon trainer would make him a valuable character. Then there's Blue, who has a connection to Red. His different approach to Pokemon training compared to Ash and Red could create some interesting storylines. And don't forget about Professor Oak. He could provide guidance and knowledge to both Ash and Red, and his presence always adds a sense of wisdom to the Pokemon world.
Including some new characters like a wedding planner who has some crazy ideas that Troy and Gabriella have to work through. And perhaps a long - lost relative of Gabriella's who shows up and has some interesting family stories to share. This would make the fanfiction more complex and engaging.