A great plot in this crossover fanfiction might be that an ancient threat from both universes resurfaces. In the HTTYD world, it could be an even more powerful and evil version of the Red Death. In the Avengers' universe, it could be a new group of super - villains. The Avengers and the HTTYD characters then have to combine their skills. The dragons could be used to transport the Avengers quickly across the HTTYD landscapes, and the Avengers could teach the Vikings about modern combat strategies.
Black Widow and Astrid would make a great pair. Black Widow is a highly trained spy and fighter. Astrid is a fierce Viking warrior. They could train together, with Black Widow teaching Astrid some stealth and hand - to - hand combat techniques from the modern world, and Astrid showing Black Widow some of the traditional Viking fighting styles and how to ride a dragon effectively.
Perhaps the vikings of Berk accidentally open a portal to Spyro's realm while exploring a new area. Spyro is intrigued by the vikings and their relationship with dragons. Meanwhile, Hiccup tries to understand Spyro's abilities and how they can be used to improve Berk's defenses. They could team up to rescue some baby dragons that were captured by an evil force from both their worlds.
The plot could involve the Vikings of Berk discovering a magical link to the world of DunBroch. Hiccup, being the curious and adventurous type, decides to explore. He meets Merida and they quickly realize that there are threats in both their worlds that seem related. Maybe a dark force is trying to use the power of dragons and some ancient magic from DunBroch. Together, they must combine their skills, Hiccup's dragon - riding and knowledge of dragons, and Merida's archery and understanding of the magic in her land to defeat this common enemy.
Well, perhaps the Infinity Stones somehow end up in the world of The Incredibles. This attracts the attention of both the Avengers and the Parr family. The two groups initially have some misunderstandings as they try to claim the Stones. But then they realize that a powerful new villain, created by the combined energy of the Stones, is emerging. So, they join forces. Dash's super - speed could be useful for quickly transporting the Stones, while Thor's lightning powers could help in battles against the villain's minions.
One possible plot could be that Oliver Queen gets transported to the Avengers' universe. He teams up with Hawkeye, sharing their archery skills and combat tactics. They could go on a mission to stop a new threat that combines the technology of the Arrowverse and the super - powered enemies of the Avengers.
A cool plot might be that the Tailed Beasts in the Naruto world are being targeted by an unknown force from the Marvel universe. The Avengers, thinking it could lead to a greater threat to both universes, come to help. Naruto and the Avengers have to work together to protect the Tailed Beasts. Hulk could have an interesting interaction with the Tailed Beasts, perhaps learning from them how to better control his rage, while the ninjas are amazed by Thor's lightning powers and try to incorporate some of that into their own jutsu.
One idea could be that Robin, with his detective skills, is investigating a mystery that has ties to the Avengers' world. Maybe it's a series of high - tech thefts that seem to be related to some new villain who has both Gotham - style cunning and super - power ambitions. Robin could team up with Iron Man to analyze the technology used in the thefts. Iron Man would be impressed by Robin's quick thinking and resourcefulness despite not having superpowers.
A possible plot could be that the Avengers are on a mission to retrieve a powerful artifact. Deadpool, who has his own reasons for wanting the artifact, also shows up at the same place. Instead of fighting each other, they form a temporary alliance. Deadpool's knowledge of the underworld and his unpredictable fighting style, combined with the Avengers' superpowers, help them overcome the various traps and enemies guarding the artifact. However, after getting the artifact, they have to decide what to do with it, which leads to some internal conflict among the group and more hilarity with Deadpool's input.
Well, perhaps a plot where the AllSpark from Transformers ends up on Earth and its energy attracts both the Transformers and the Avengers. The Avengers are initially suspicious of the Transformers. But as they learn about the Decepticons' plan to use the AllSpark for evil, they join forces. The Hulk could even have some interesting interactions with the larger Transformers like Grimlock.