Well, one possible plot could be that Hiccup accidentally drinks a magic potion made by a mischievous wizard. As a girl, he (now she) has to deal with the different reactions of the people in Berk. Some are shocked, some are curious. And she has to learn how to adapt to the new body while still maintaining her heroic spirit, facing the same dragons but in a different physical form.
In a 'hiccup turns into a girl' fanfiction, the plot might start with a strange curse. Hiccup wakes up one day as a girl. At first, he's really confused. Toothless might not recognize him right away. Hiccup then tries to find a way to reverse it. Along the way, he discovers new things about himself. He might find that as a girl, he has different strengths and weaknesses. He could also have some interesting interactions with Astrid, who is both confused and intrigued by this new version of Hiccup.