Maybe Mavis, in her exploration of different realms, ends up in the ninja world. She gets into trouble and Naruto comes to her rescue. Naruto is intrigued by her magic and Mavis is impressed by his ninja skills. This encounter plants the seed for their relationship that eventually leads to marriage.
One possible plot could be that there is some sort of misunderstanding or dark magic at play that causes Natsu and Mavis to fight. Natsu, being hot - headed, jumps into the fray without fully understanding the situation. Mavis, on the other hand, is trying to figure out a way to stop the fight without harming Natsu. As the battle progresses, they both start to realize that they are being manipulated and then team up to face the real enemy.
In a fight between Natsu and Mavis in a fanfiction, it's really hard to say who would win. Natsu is extremely powerful with his dragon slayer magic, being able to take on some really tough foes. However, Mavis is the first master of Fairy Tail and has a great deal of wisdom and powerful magic herself. It could really go either way depending on how the fanfiction writer wants to portray the battle.
Naruto has shown incredible feats of strength, speed, and endurance. He has defeated some of the most powerful villains in his universe. But Mavis is no slouch either. Her Fairy Law is a very powerful spell. If we consider their growth potential, Naruto has the ability to constantly get stronger by unlocking new forms of his power, like Sage Mode and later the forms related to the tailed beasts. Mavis, on the other hand, has a deep knowledge of magic that could potentially allow her to counter any of Naruto's attacks. It really depends on the situation, the location, and how well they know each other's abilities.
One of the main characters could be Wayne, the werewolf. He and his family could bring some fun and chaos to the story. Maybe they come to visit Johnny and Mavis, and the werewolf pups accidentally cause some trouble in the baby - proofing of the house. Also, Griffin, the invisible man, could be there. He could use his invisibility to play pranks on the couple during their pregnancy, but also be a good friend when they need help.
Their relationship would likely become more centered around the baby. They would have less time for just themselves and more time taking care of the baby's needs.
Well, in many fanfictions, their relationship might start with Natsu being a bit in awe of Mavis as she is a very important figure in Fairy Tail's history. Natsu, being his brash self, might initially make mistakes around her but gradually learns from her wisdom. As time goes on, he could come to respect her even more deeply and they might form a bond where Mavis becomes like a guiding light for Natsu in his adventures and growth within the Fairy Tail guild.
One idea could be that in the fanfiction, Mavis discovers she's pregnant unexpectedly. Her friends and family are in shock at first but then rally around to support her. Maybe the father of the baby is a mysterious character, adding an element of intrigue to the story.
I'm not entirely sure if there is a specific well - known fanfiction with Naruto as Mavis' father. There are so many fanfictions out there, and it could be a very niche or new concept. However, given the creativity of fanfiction writers, it's not impossible that someone has written a story with this premise. Maybe it involves some kind of magical or time - traveling plot device to make such an unlikely connection between the two characters from different universes.