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.
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.
Look to Chapter 4 for the clearest answer. It begins with Qiao regretted her words as soon as they left her mouth., keeps building through Whether Jiang Yan can be found or not was another matter., and lands on A little girl acting mysterious, learning fortune-telling, what is this....
Chapter 9 is the strongest chapter-level answer because it starts from Exactly, the Qiao Family isn't that great either. and follows through on Qiao, unable to save face, slapped her daughter across the face, "Didn't I....
Look to Chapter 36 for the clearest answer. It begins with But soon, people began to question the authenticity of Jiang Li's bare..., keeps building through After all, these days, faux bare-faced makeup can be indistinguishable..., and lands on After swallowing the food in his mouth, he hurriedly.
Chapter 744, "Dig Three Feet into the Ground to Find Her!", is the clearest early answer because it turns Xing Mingfei's situation into a concrete plot event. A useful checkpoint is the way the scene starts with What?!" Wen Jiaojiao directly questioned out loud, "On what basis are...
If you want the clearest chapter reference, go to Chapter 10. Early on, Qiao's shocked face struggle to recover, Lin Manru also tasted for the...; later, the chapter leans on Jiang Li nodded with a smile, seemingly approving of Lin Manru's words and....
Chapter 39 is the strongest chapter-level answer because it starts from Qi Tianyu had been looking forward to watching Jiang Li's drama unfold. and follows through on His original intention was just to make Jiang Li back off gracefully,....
The best chapter pointer here is Chapter 1, "I'm Not Interested in Uncultured Men". It opens with Jiang Li, I'm warning you, don't set your sights on Yinwan anymore, or... and closes on Yet before she had the chance to teach her a lesson, the male lead arrived....
Chapter 932, "Gu Pansheng [Extra · Main Character Chapter]", is the clearest early answer because it turns Hua University's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Gu as in 'Gu Pansheng,' Pan as in 'Gu Pansheng.' I am four years old...
Chapter 10 is the strongest chapter-level answer because it starts from Qiao's shocked face struggle to recover, Lin Manru also tasted for the... and follows through on Who was it again saying that the Jiang Family's tradition was lacking,.... The payoff is that the chapter closes with "She...