The connection between the fanfiction and the trailer is often loose. Fanfiction writers might use elements from the trailer as a starting point but then build their own unique stories and character developments.
Well, Mugen Visual Novel is a concept that fuses two different things. Visual novels are known for their story - driven gameplay, with lots of text, choices, and different story paths. Mugen, on the other hand, has been associated with creating custom fighting game content. In a Mugen Visual Novel, perhaps you could have a story where the outcome is affected by fighting elements. For example, characters might battle to progress the story, or their fighting abilities could be part of their character development in the narrative.
You can try some popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just search for 'How to Train Your Dragon' and include 'trailer' in the search terms to narrow down the results.
A key feature of mugen games with story is the character development. Since there are often a large number of characters from different sources, the story allows for their growth and evolution. Another feature is the plot progression. There's usually a main goal or conflict that drives the story forward, like in "Mugen Souls" where Chou - Chou's quest to rule the multiverse is the central plot. Also, the story can add replayability as players may want to experience different story paths or outcomes.