Well, perhaps there's a dark mage who creates a new kind of curse. Natsu is the unlucky target and is transformed into a baby. Gray would probably be both amused and worried. They would have to search for a rare magical item to turn Natsu back. Meanwhile, baby Natsu might start to bond with the guild members in a different way, like clinging to Erza and making her more maternal than her usual tough self.
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.
Maybe there's an ancient artifact in the guild's basement that Natsu accidentally activates. It backfires and turns him into a baby. The guild then has to work together to find a solution. Happy could act as a sort of guide for baby Natsu, trying to communicate with him in his own way. Natsu as a baby might also show some new, innocent sides of his personality, like being fascinated by simple things like shiny objects or the guild's pets.
Chapter 719, "Borrowing Their Luck", is the clearest early answer because it turns Chang Song's situation into a concrete plot event. The chapter gives readers a real answer by opening on Chang Rong: "Hmph, it has nothing to do with me anyway, since no one...
Chapter 740, "Someone Who Can Live Through the Days", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with Fang Kai sprang to life when he heard this, immediately setting his...
Chapter 2 is the strongest chapter-level answer because it starts from On the top floor of the Financial Building, a gathering of dark clouds... and follows through on Jiang, Lin Manru, turned pale and quickly explained, "It's not eloping, my....
Chapter 690, "Miss Jiang, Marry Me!", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. Early on, No wonder she had predicted she would come into some unexpected.... By the end, Rong Kexi, however, glanced toward the door, irritation flaring in....
Look to Chapter 8 for the clearest answer. It begins with The resentment that had barely calmed among the crowd surged back two..., keeps building through Some of them were relatives of the Jiang Family, who felt it improper to..., and lands on Other people with a handsome and charming appearance.
If you want the clearest chapter reference, go to Chapter 13. Early on, Sensing that something was off with Jiang Yan, Jiang Yan dismissed the...; later, the chapter leans on What on earth do you mean?" Ignoring him once again, Jiang Li shifted her....
Chapter 751, "Even if both perish, he must pay the price!", is the clearest early answer because it turns Fang Wenzhe's situation into a concrete plot event. A useful checkpoint is the way the scene starts with She cried out for heaven and earth, but her beloved was just...
Chapter 708, "Stinky Beggar", is the clearest early answer because it turns Wu Daoshan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Chen'an's executives feared that the situation would escalate and...
Chapter 585, "He, Gu Yu, really has no bottom line!", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. The chapter gives readers a real answer by opening on The melee by the lake gradually subsided after an hour.
Chapter 874, "All the Same", is the clearest early answer because it turns Shi's situation into a concrete plot event. The chapter gives readers a real answer by opening on This family is surnamed Shi, and they have lived in this village for...