Their friends in the guild would likely be very supportive. Happy would probably be over the moon, flying around them and chirping happily. Erza might give them a stern but warm look and offer some words of advice about maintaining a healthy relationship. Gray would probably make some snarky comment at first but deep down be happy for them. And Wendy would be all smiles, seeing two of her favorite people find love with each other.
As they travel, their bond deepens. They start to rely on each other more. Natsu becomes more protective of Lucy, and Lucy begins to understand Natsu's impulsive nature better.
I think characters like Makarov, the guild master, would be there. He could give a really heartwarming speech about how he's seen Natsu and Lucy grow. Wendy and Carla would also be guests. Wendy might be helping with some of the decorations, using her magic in a creative way, and Carla would be there to keep Happy in line and also enjoy the celebration.
Maybe the story could start with Natsu and Lucy going on a special mission. During this mission, they face a great danger that makes them realize how much they mean to each other. Once they return, Natsu decides it's time to get married. The fanfiction could then focus on the wedding preparations, like Lucy choosing her dress and Natsu trying to find the perfect gift for her.
There might be some fanfictions that depict Natsu and Lucy sleeping together in a non - inappropriate way, like they could be sharing a room during a quest out of necessity and just sleeping in separate beds or something similar. But it can be hard to find ones that are completely pure in the vast world of fanfiction.
One interesting adventure could be when they end up in a parallel dimension. In this new world, they have to figure out how to get back home. Along the way, they meet strange creatures and learn new magic spells that are unique to that dimension.
Natsu and Lucy don't tie the knot in the manga. The plot mainly centers around their friendships and the challenges they face together, not their marital status.