Well, a plot could revolve around them starting a family. Hiccup and Heather could face difficulties in having children, like dealing with some sort of magical or environmental factor that is affecting their ability to conceive. Or perhaps they adopt a child from a troubled tribe and have to help the child adjust to their new life with them.
A really interesting plot might be that Hiccup and Heather get married in a hasty ceremony due to some external threat. Then, they have to learn about each other on a deeper level as they face this threat together. Maybe the threat is a new dragon species that is much more powerful and intelligent than any they've faced before, and they have to combine their skills and knowledge to defeat it and at the same time strengthen their marriage.
One plot idea could be a story where Hiccup, Heather and Lemon are on a quest to find a rare dragon. Along the way, they face various challenges that test their friendship and skills. Hiccup's dragon training knowledge, Heather's combat skills, and Lemon's unique problem - solving abilities all come into play as they battle the elements and other dragon - trainers who want the dragon for themselves.
A really interesting plot could be them starting a family. How they raise their children with the combined values of their cultures would be fascinating. Merida's archery skills and Hiccup's dragon - taming abilities could be passed down in unique ways, and they'd have to deal with the joys and difficulties of parenting in a world that is still somewhat unfamiliar to their combined family.
The plot could be about Hiccup and Heather going on a quest while astride some sort of creature or vehicle, and the lemon might be a key to unlocking something along the way.
A powerful dragon rider from a far - off land who has a grudge against the Berk tribe (Hiccup's tribe) could be the main antagonist. He or she sees Hiccup and Heather's marriage as a symbol of the growing strength of Berk and decides to target them specifically to weaken the tribe. They might try to steal their dragons or spread false rumors about their marriage to create unrest among the people.
One idea could be that Heather is a new ninja from a far - off land who comes to Konoha. Naruto is immediately intrigued by her unique fighting style. They start training together and gradually develop feelings for each other as they share their ninja dreams and experiences.
A possible plot could be that they both enter a local competition, perhaps a writing or a music competition. Charlie Brown is hesitant at first because he's used to not winning. Heather encourages him and they start to practice together. As they work together, they start to understand each other better. Their different personalities - Charlie Brown's pessimism and Heather's optimism - complement each other and they create something truly amazing for the competition.
Another plot could involve the main character in the fanfiction realizing that they got married too young and now they are having second thoughts. This could lead to a series of self - discovery and relationship - testing events. For example, they might meet someone new who makes them question their marriage, but in the end, they realize the value of their relationship.
A possible plot is that there is a magical event that switches the minds of Stoick and Baby Hiccup. So Stoick has to figure out how to take care of himself as a baby while Hiccup, in Stoick's body, has to deal with the responsibilities of being the chief. It would be full of comical and heartwarming moments as they both learn about each other in a very different way.
One idea could be Hiccup discovers a hidden tribe in a remote jungle. Another could be that he encounters a mythical creature in an uncharted ocean region. Maybe he also teams up with a group of international dragon riders to save a world - spanning dragon sanctuary.
One idea could be that Hiccup discovers a new type of half - dragon that has unique abilities, like the power to control time briefly. This could lead to adventures where he has to use this new discovery to save Berk from an ancient threat that is about to be unleashed. Another plot could involve the half - dragon being a hybrid of a very rare and powerful dragon species, and Hiccup has to protect it from dragon hunters who want to use its power for evil.