A powerful demon might be the main antagonist. The demon senses the pure and strong magic within the baby and wants to corrupt it. Natsu and Lucy would need to use all their skills and the help of their friends to fend off the demon. This would also test their relationship as they're responsible for the safety of the baby.
Perhaps an organization that believes the baby is a threat to their plans. They might think the baby's existence could disrupt the balance of magic in some way, so they send their agents to capture or eliminate the baby. Natsu and Lucy would then have to fight these agents to protect the baby. They'd face various challenges and dangerous foes within this organization.
It could be a dark wizard who wants to use the baby's power for his own evil deeds. Maybe the baby has some special magic that the wizard thinks can help him take over the magical world.
It could be a new, powerful dark wizard who has a grudge against Fairy Tail. This wizard might manipulate Natsu into kidnapping Lucy to create chaos within the guild. The dark wizard could be after some powerful magic that only Lucy can access, and by kidnapping her, they hope to force her to give it up.
It could be another dark mage who sees the potential in baby Natsu and wants to use him for evil purposes. This mage might view Zeref as an obstacle in getting to Natsu.
Maybe a rival guild that is jealous of Fairy Tail's new addition. They might think that having a baby with such strong magical potential in Fairy Tail gives them an unfair advantage. So they try to kidnap the baby to either keep it for themselves or use it to bargain with Fairy Tail.
Well, perhaps in this fanfiction, Natsu calling Lucy 'baby' is a sign of a new level of closeness in their relationship. It could start with Natsu protecting Lucy in a dangerous situation and instinctively calling her 'baby' as he holds her close. This then leads to Lucy being a bit flustered at first but gradually getting used to it and even liking it, and it becomes a special part of their bond.
Well, perhaps the story could start with Lucy finding out she's pregnant. Natsu is initially shocked but then becomes super excited. The pregnancy is filled with little adventures, like Natsu trying to build a perfect nursery using his magic in a rather clumsy way. And when the baby is born, it brings a new sense of hope and family to the whole guild. Everyone pitches in to take care of the baby, and it grows up surrounded by love and magic.
Growth could also be a theme. Nashi would be growing up, learning about her powers (if she has any), and about the world around her. Natsu and Lucy would also be growing as parents, learning how to take care of a child, make decisions for the family, and balance their previous lives with their new roles as parents.
One possible plot could be that Natsu and Lucy accidentally end up taking care of a baby dragon. They struggle at first with things like feeding it and getting it to sleep. But as time goes on, they grow attached and learn a lot about themselves and their relationship in the process.
Most likely. When one character calls another 'baby' in a fanfiction, it often implies a certain level of romantic affection. So, it's a good bet that the 'natsu calls lucy baby' fanfiction has romantic themes.
One possibility is that the baby is a key to a mystery. Maybe it has some special power or mark, and villains are after it. Natsu and Lucy then take on the role of protectors, using their magic and friendship to keep the baby safe while trying to figure out its origin.
In the 'natsu and lucy baby nashi fanfiction', we can expect a lot of heartwarming scenes. Natsu and Lucy would surely have to adjust their lives. Nashi might inherit some of their powers or traits. There could be a story arc about Nashi growing up and wanting to follow in her parents' footsteps. The fanfiction might also show how their friends in the story react to the new addition to the family, and how they all contribute to Nashi's upbringing.