Perhaps the baby is related to one of the main characters. It could be the child of a character who has lost their memory or been in some sort of danger. So the discovery of the baby at the doorstep kicks off a journey of self - discovery for that character within the fanfiction.
One possibility is that the baby could be a key to a long - forgotten prophecy in the Fairy Tail universe. Maybe it has special powers that the villains in the fanfiction are after, and the Fairy Tail guild has to protect it.
The baby can introduce a new emotional aspect. The characters in Fairy Tail are known for their strong emotions, and the presence of a helpless baby can bring out their protective and nurturing sides, changing their relationships with each other.
He could be a new addition to the Fairy Tail guild. As a baby, he'd bring a lot of innocence and new energy. The guild members would have to take care of him, like how they take care of each other. Maybe he'd be found near the guild's entrance one day, and they'd decide to keep him. His ninja abilities, even at a baby stage, might start to show in small ways, like his speed or reflexes, which could be interesting to the magic - using guild members.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
The condition could be that the main character has to give up their magic for a period of time. This would create a lot of interesting plot twists as they'd have to rely on their wits and the help of their friends in the Fairy Tail guild instead of their magical abilities.
In fanfiction, their kiss could symbolize a moment of forbidden love. Zeref, with his dark past and power, and Mavis, the founder of Fairy Tail. Their kiss might be a turning point where they bridge the gap between their different natures. It could be a moment of pure emotion, defying all the odds that keep them apart in the original lore.
One idea could be exploring the daily lives of the guild members when they're not on official quests. For example, how they train in secret or their personal hobbies. Another could be about the backstories of some of the minor characters that we don't get to see much in the main series.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.
In the Fairy Tail world, Naruto would bring his unique ninja abilities. For example, his shadow clone jutsu could be used in creative ways during battles or to perform tasks. He would likely befriend Natsu because they both have a passion for fighting and protecting their friends. Naruto could also introduce new combat styles to the Fairy Tail guild, which might change the way they approach battles.
Well, the story could be full of heartwarming moments. The baby might inherit Natsu's fiery spirit and Erza's strong will. They could raise the baby in the Fairy Tail guild, surrounded by friends like Lucy and Gray. Maybe the baby would have some unique magic abilities, a blend of Natsu's dragon slayer magic and Erza's requip magic. And the guild members would all chip in to take care of the little one, with comical and touching scenes of babysitting.
One idea could be that Baby Naruto somehow gets transported to the Fairy Tail world. He could end up being taken in by one of the guild members, like Lucy. Since he's a baby, he'd be super cute and the members would be protective of him. They might even try to train him in their magic, which could lead to some really interesting and funny situations as Naruto tries to adapt to this new world of magic.