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.
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.
There could be a 'fairy tail baby fanfiction' where a time - traveling baby arrives in the Fairy Tail world. This baby has knowledge of future events and the characters have to figure out how to deal with this. Some might be skeptical, while others like Lucy might be more open - minded and try to understand the baby's cryptic messages.
First, know the characters of Fairy Tail well. Their personalities should shine through even when dealing with a baby. For example, Gray's cool - headedness and Erza's strong leadership. Second, create an interesting backstory for the baby. Maybe it's from a lost civilization or has a special connection to one of the main characters. Third, add some action and adventure. Don't just focus on the baby being cute, but also on how it changes the world of Fairy Tail.
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.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'Fairy Tail' fanfictions, and you're likely to find some great 'Nalu baby' ones there.
One great 'Fairy Tail Nalu baby' fanfiction could be the one where Natsu and Lucy's baby has special magic abilities inherited from both of them. It shows how they struggle and learn to raise the baby while dealing with the challenges in the Fairy Tail guild.
Well, you could write about Nashi's early days in the context of Fairy Tail's world. For example, how does she adapt to the noisy and lively guild environment as a baby? Maybe she loves the music that some of the mages play and it has a strange calming effect on her. Or perhaps there are some other babies in the guild at the same time, and their interactions could be really cute and interesting.
Their relationship can become more nurturing. Natsu, who is usually brash and impulsive, might become more gentle and protective around the baby. Lucy, on the other hand, would be the more practical one, making sure the baby has everything it needs. This would make Natsu see Lucy in a new light, as a motherly figure, and he would respect her even more. Their love for each other could also deepen as they share the joys and challenges of raising a baby.