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.
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.
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.
Look to Chapter 33 for the clearest answer. It begins with Following the announcement, five artists respectively reposted the Weibo—..., keeps building through Instantly, "Shining Transformation Trip," already under intense scrutiny,..., and lands on Although Su Yinwan was considered half a.
The best chapter pointer here is Chapter 36, "The Coldest Variety Show Opening in History". It opens with But soon, people began to question the authenticity of Jiang Li's bare... and closes on After swallowing the food in his mouth, he hurriedly took his megaphone....
Chapter 720, "A Bastard Child Behaving So Arrogantly?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 893, "From today onwards, he is my half-brother from another father and mother!", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with Jiang Yan still had unwavering faith in Daniu's character.
Chapter 920, "College Entrance Exam Results Inquiry", is the clearest early answer because it turns Meng Xuezhi's situation into a concrete plot event.
Chapter 33 is the strongest chapter-level answer because it starts from Following the announcement, five artists respectively reposted the Weibo—... and follows through on Although Su Yinwan was considered half a goddess to him, the whole thing....
If you want the clearest chapter reference, go to Chapter 8. Early on, The resentment that had barely calmed among the crowd surged back two...; later, the chapter leans on Some of them were relatives of the Jiang Family, who felt it improper to....
Chapter 826, "Deadlock", is the clearest early answer because the violence around Mr becomes final instead of hypothetical. It opens with The Li Family Master had spoken, so Father Yu quickly followed suit... and closes on Yu looked puzzled, "What, you all know this Jiang Li?" Father Yu had....
A practical answer is Chapter 32, because it turns the idea into a concrete scene. The chapter opens with You should know, an ID photo is like a mirror for demons., adds pressure through This time, we bring six guests to travel to the distant Highland Mountain..., and ends on With airflow comes.