There could also be some of Hermione's former classmates from Hogwarts that we don't know as well. Maybe some Ravenclaws or Hufflepuffs who she studied with and became friends with over the years. And of course, some of the Order of the Phoenix members who were still around could have been invited to share in the joy.
One possible plot could be that it focuses on the friends and family reminiscing about Hermione's wedding 20 years ago. Maybe they gather for an anniversary celebration and share stories that were never told before, like how nervous Hermione was right before the ceremony or how Ron accidentally spilled something on his suit.
Professor McGonagall might be an honored guest. She has seen Harry and Hermione grow from students to heroes of the wizarding world. Her presence would be a symbol of the connection between their past at Hogwarts and their new future. And Dumbledore's portrait, if it has the ability to interact with the living on such an occasion, could also be considered a special guest, as he has played such a significant role in their lives.
One great Hermione 20 - years - back fanfiction is 'Hermione's Time - Warp Adventure'. It takes Hermione back in time and has her navigate through the wizarding world of 20 years prior. The story does a great job of showing how Hermione's modern knowledge clashes with the older ways of magic.
In this fanfiction wedding, not only their friends but also some of the heroes they've worked with in the past could be guests. Rena Rouge and Carapace would be there, having played important roles in many battles. Maybe even some of the one - time heroes like Pegasus or Viperion could show up. Ladybug's parents would be there with open arms, and Adrien's bodyguard could also be present, having seen Adrien grow up. And Paris itself could be considered a 'guest' in a way, as the city has been a big part of their lives and the reason for their superhero existence.
Obviously, their close friends from Hogwarts would be there. Ron and Hermione would be among the guests, as they are Harry and Ginny's best friends. The Weasley family in its entirety would be present too, including Molly and Arthur. Luna Lovegood would surely be invited, with her unique charm. And Hagrid, who has been like a father figure to Harry in a way, would also be there to celebrate this happy occasion.
Definitely their friends from the Pokémon journey. People like Brock, Clemont, and Bonnie would be there. All of Ash's Pokémon from his various travels, like Charizard, Bulbasaur, and Squirtle, would also come. And Serena's Pokémon friends too. Maybe even some of the Gym Leaders they've met over the years would show up to wish them well.
The guests would include their closest friends and allies. Nico would be there, having finally come to terms with Percy and being part of their group. Rachel Elizabeth Dare would be there as well, since she has a connection to both of them. The centaurs from the forest around the camp would also be invited, like Firenze, who has always been a bit of a mentor figure. And the minor gods and goddesses who have helped them on their adventures might also make an appearance.
Obviously, Brock would be there. He's been their friend throughout their journey. All of Ash's Pokémon like Pikachu, Bulbasaur, and Charizard would be present too. And Misty's sisters might come to the wedding, adding some fun and chaos as they usually do.
Their families would definitely be there. So Katniss's mother and sister Prim, and Peeta's family. Also, their friends like Gale, who, despite his complicated relationship with Katniss, would come to support them. Haymitch would also be a guest, as he has been a mentor and a friend to them throughout their time in the Games.
Well, it often creates a lot of interesting situations. For example, Hermione's knowledge from the future can be a huge advantage or a source of trouble. In some stories, she might use her advanced magic knowledge to impress others, but in others, it might make people suspicious of her.