The other Vikings would see Hiccup in a whole new light. Some might be scared of him as a baby Night Fury, thinking he could be dangerous. But those who are closer to Hiccup, like his father (if he figures it out), would be more worried and try to find a way to reverse the transformation.
With Astrid, it would be really different. Astrid would probably be very confused at first. She might be a bit more cautious around Hiccup as a baby Night Fury, not knowing it's him. But as she figures it out, she would likely be very determined to help him turn back.
His relationship with Toothless would change drastically. Instead of being the rider, he becomes like a little sibling or a young apprentice to Toothless. Toothless would have to be more protective and patient with Hiccup.
His relationship with Toothless would be very different. Toothless might be more protective of him in his new form, as if sensing that Hiccup is more vulnerable as a female Night Fury. And Hiccup would have to communicate with Toothless in new ways, perhaps using more body language that is typical of female Night Furies.
His relationship with Toothless would become more complex. They would now be of the same species, which could lead to either competition or a deeper bond. Maybe Toothless would initially be confused about Hiccup's transformation but then accept him as an equal. They might communicate in new ways, without the need for Hiccup's human - made gadgets.
Inuyasha's relationship with his enemies would also change. Some enemies might underestimate him in his baby form, not seeing him as a threat. However, some might see it as an opportunity to get rid of him once and for all. His friends would have to be extra vigilant in protecting him. And as Inuyasha grows up again in the story, his relationships with his friends would be different because of this experience. They would have a shared memory of taking care of him when he was at his most vulnerable, which could deepen their loyalty and friendship.
His relationship with MJ would become very different. Instead of a romantic partner, she might become more of a protective caretaker. She would have to deal with a baby - like Peter, which could be both challenging and endearing.
With his human friends, it would be really tough. They might be scared of him at first. Astrid, for example, would be conflicted. She was attracted to Hiccup as a human, but now he's a dragon. She would have to overcome her fear and try to find a way to communicate with him. And Hiccup would feel isolated from his human friends, longing for the connection they once had.
For Frisk, they might see Sans in a new light. Instead of the cool and mysterious Sans they know, he's now a helpless baby. Frisk could become very protective, using their determination to keep baby Sans safe. And the monsters in the Underground would likely be more gentle and patient around baby Sans, changing the overall dynamic of the community.
One idea could be that Hiccup wakes up one day to find himself in the body of a female Night Fury. At first, he's completely confused and tries to figure out how to control this new form. He might struggle with flying at first, but as he gets used to it, he discovers new abilities that come with being a female Night Fury, like a different type of sonic blast. The other dragons in the village are initially startled but then start to accept him in this new form.
His relationship with Chi - Chi would be very different. Instead of being her husband, he's now like a son to her. She would be super protective. And with Gohan, it would be more like they are brothers. Gohan would have to look out for his 'baby father'.
His relationship with Toothless would be very different. Toothless might be initially confused and a bit hesitant around Hiccup in his new form. But as time passes, they would develop a new kind of bond based on Hiccup having to adapt to being a dragon. With Astrid, it could be strange at first. Astrid might be both curious and a bit jealous of Hiccup's new dragon form. She would have to see Hiccup in a whole new light.
Well, with Sasuke, their relationship would become more of a protective - protectee one. Sasuke, who usually has a rivalry with Naruto, would now feel responsible for protecting this baby Naruto. With Sakura, she would be more motherly towards him. She might be the one constantly fussing over him, making sure he's clean and fed.