Lucy would be very affected. She's one of Natsu's closest friends. She'd have to take on a lot of responsibility for taking care of baby Natsu, which would disrupt her normal life of doing quests and writing her novel.
One possible plot could be that the magic in the world goes haywire. Natsu is caught in a spell that turns him into a baby. His friends in Fairy Tail are shocked at first. Lucy might take on the role of a caretaker, trying to figure out how to reverse the spell while also dealing with a baby Natsu's unpredictable behavior like randomly using small bursts of fire magic that are more like cute little puffs now.
One idea could be that in this fanfiction, Wolverine turns into a baby due to a powerful mutant's accidental spell. The X - Men then have to take care of him while also trying to find a way to reverse the transformation. They might discover new aspects of Wolverine's personality as a baby, like his unwavering determination is now shown through his attempts to crawl towards danger.
In some fanfictions where Natsu turns into a dragon, it often gives him extraordinary power. He might have enhanced fire abilities, being able to breathe more intense flames. His physical strength could also increase exponentially, making him an even more formidable force in battles. Maybe he would also gain some dragon - like instincts, which could either help him in combat or cause some internal conflict as he tries to balance his human side with his new dragon self.
If Natsu turns into a girl in fanfiction, his appearance would be quite a change. He might have long pink hair, and his eyes could be even more expressive. His usual dragon - slayer outfit would have to be adjusted to fit his new female form, perhaps becoming more form - fitting or having some feminine touches. In terms of his magic, it would remain the same, but the way he uses it could be different. He might be more strategic, using his new - found grace to his advantage in battles.
Another theme might be how the other characters in the story react to Natsu's transformation. Their shock, acceptance, or rejection can form an important part of the plot. For example, some friends might be very supportive while others could be confused or even hostile at first.
One idea could be that a villain's new invention accidentally zaps Kim and turns her into a baby. Her sidekick Ron has to take care of her while also trying to find a way to reverse the effect. He might struggle with basic baby - care tasks like changing diapers and feeding her, all while keeping her safe from the villain who might want to take advantage of her weakened state.
One possibility could be that an akuma's power goes wrong and instead of attacking Chat Noir in a normal way, it turns him into a baby. Ladybug is then left to protect this baby Chat Noir while also trying to find a way to reverse the transformation. She might have to take him to Master Fu for help, all while hiding him from Hawk Moth who could potentially use this situation to his advantage.
One great fanfiction with this concept is 'Baby Dean's Adventures'. It has a really cute take on how the other characters deal with a baby Dean. They struggle at first but then it becomes a heartwarming story of protection and family.
There could be a powerful demon from another dimension. As Natsu becomes an adult, his magic signature becomes stronger, which attracts this demon. The demon wants to possess Natsu's power for itself and will stop at nothing to achieve this goal. It would be a formidable foe, with abilities that Natsu has never faced before.