Well, there's 'Hiccup's Dragon Heritage'. In this fanfiction, Hiccup discovers that his dragon - like traits are linked to an ancient prophecy. It's really interesting as it shows how he has to learn about a whole new side of himself. Also, 'Dragon - Part Hiccup' is a great one. It has a lot of action scenes where Hiccup uses his unique abilities to fight off new threats to Berk.
One popular 'hiccup is part dragon' fanfiction might be 'Dragon's Bloodline'. It delves into how Hiccup's new dragon - like abilities change his relationship with Toothless and the other vikings. Another could be 'The Half - Dragon Hiccup' which focuses on his struggle to control his new powers while still being a leader in Berk.
A really engaging 'hiccup is part dragon' fanfiction is 'The Dragon Within Hiccup'. It starts with Hiccup having strange dreams about being a dragon and then slowly realizes he has some dragon - like characteristics. The story is beautifully written, with detailed descriptions of how Hiccup feels about this new part of him. There are also some great character interactions, especially between Hiccup and Astrid as she tries to understand what's happening to him.
In fanfictions with 'hiccup is part dragon', it completely transforms the power dynamics. Hiccup now has abilities like flying without Toothless or breathing a small amount of fire. This changes how he approaches battles and threats. He might be seen as a freak by some vikings, which leads to him having to prove himself even more. It also means his bond with Toothless could be different, as they are now more alike in a way.
One popular fanfiction might be those that focus on new adventures for Hiccup and the Dragon Army. For example, they could go on a quest to discover a new type of dragon that has unique abilities. Another could be about how they deal with a new threat to Berk that is more powerful than any they've faced before. There are also fanfictions that explore the relationships within the Dragon Army in more depth, like the friendship between Hiccup and Astrid as they lead the group.
One popular 'hiccup has dragon eyes' fanfiction might involve Hiccup using his dragon - like eyes to see hidden treasures in the dragon world. He could discover ancient dragon artifacts that were previously unknown.
There is a fanfiction titled 'Hiccup and Ruffnut: Dragon Riders' Allies'. In this one, they have to work together to defend Berk from a new threat. It's full of action scenes and also some really interesting character development for both of them.
Some fanfictions that are worth reading are 'Under the Northern Lights'. It has a very romantic setting where Hiccup and Astrid share a special moment. 'Hearts Entwined' is another one where their relationship is explored in great detail, from the first flutters of attraction to a deep, abiding love. 'The Love that Soared' also shows their love growing as they face challenges together in Berk.
One good Merida x Hiccup fanfiction is 'A New Alliance' which explores their journey together in a world where their kingdoms need to unite. It has great character development.
Another popular one could be 'Hiccup and Astrid's Unseen Bond'. It delves deep into their relationship, exploring the emotional connection that might not be as obvious in the original series. It shows how they support each other through tough times and how their love blossoms over time.
One great 'Toothless loves Hiccup' fanfiction is 'Bond Beyond Words' on FanFiction.net. It beautifully portrays their relationship with lots of heartwarming moments between the two.
One popular 'hiccup leaves berk' fanfiction could be about him leaving to explore new lands and discover unknown dragon species. It might focus on his solo journey and how he overcomes various challenges without the support of his Berk community.
I'm not sure about specific 'dragon hiccup yaoi fanfic' titles off the top of my head. But you can try searching on fanfic platforms like Archive of Our Own. They have a wide range of fanfics and you might be able to find something related to your rather unique theme there.