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.
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.
Another fanfiction could start with Merida getting lost in the Viking lands. Hiccup finds her and at first, they have a bit of a misunderstanding because of their different cultures. But as they spend time together, Hiccup helps Merida understand the ways of the Vikings and Merida teaches Hiccup some of her own skills, like how to better track in the forest. Eventually, they become good friends and help each other out in various adventures in the Viking world.
Maybe it turns out that Merida's magic has a connection to the dragons in Hiccup's world. When she first arrives in his land, she accidentally awakens an ancient and powerful dragon that was thought to be just a legend. This dragon is both a danger and a key to solving a bigger mystery that has been plaguing Hiccup's village. And Merida has to learn to control her new - found connection to the dragon with Hiccup's help.
Well, you can start by looking on popular fanfiction websites like Archive of Our Own. There might be some great stories there about Hiccup and Merida being married. However, I haven't read a specific one myself to recommend directly.
The plot could center around a competition. Hiccup is invited to Merida's land for a friendly contest. They both enter different events. Hiccup in a dragon - riding competition and Merida in an archery tournament. Along the way, they learn from each other's strengths and overcome their own weaknesses. They also have to deal with a cheating competitor who is trying to undermine the spirit of the competition.
One possible plot could be that Merida and Hiccup meet at a grand gathering of different kingdoms. At first, they are a bit wary of each other. But as they start to talk about their shared love for adventure and their experiences with their respective mythical creatures (Merida with her bear and Hiccup with Toothless), they begin to form a connection. They then go on a quest together to save both their lands from a common threat, and during this time, their relationship deepens into love.
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.
Well, perhaps there could be a fanfiction where Hiccup and Elsa are both summoned by a powerful wizard to help with a prophecy. They don't trust each other at first, but as they face the challenges set by the wizard, which include tasks that require both fire (from Hiccup's dragon) and ice (Elsa's power), they start to respect and care for each other.