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.
Well, perhaps there's a threat that can only be defeated by combining the chakra in the Naruto world with the superpowers of the Avengers. For example, Thor's lightning could combine with Sasuke's lightning - based attacks. And Captain America's leadership could inspire Naruto to lead his own team in a new way. They all start off not trusting each other much, but as they face challenges together, they learn to work as one.
One possible plot could be that the Avengers are somehow transported to the Naruto universe. They team up with Naruto and his friends to fight a common enemy like Kaguya who has somehow obtained the Infinity Stones. The Avengers' powers and technology would be a huge shock to the Naruto world, and they'd have to learn to combine their skills to succeed.
One possible plot could be that Naruto is somehow transported to the Marvel universe. The Avengers initially mistake him for a threat. But Naruto, being Naruto, tries to explain his situation. He then shows them his ninja skills like shadow clones and Rasengan. Together, they could face a common enemy like Thanos, who has acquired some sort of ninja - like powers. Naruto's ability to never give up and his strong will could inspire the Avengers, and his combat skills could be a great addition to their team in the fight.
Well, a great plot could be that the Infinity Stones start to mess with the fabric of reality and it creates a portal between the Naruto universe and the Marvel universe. The Avengers and Naruto's friends end up in each other's worlds. Naruto and the Avengers have to work together to fix the portals and return everyone to their proper universes. Along the way, Naruto might have some epic battles with villains like Loki, using his ninja speed and strength. And the Avengers could learn a thing or two about chakra from Naruto and his friends.
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.
Perhaps the Joker discovers a way to mind - control some of the Avengers. He targets Hulk first, making him go on a rampage in a major city. The other Avengers are caught off - guard. They soon realize it's the Joker's doing. Spider - Man, who has recently joined the Avengers, swings in to help. He uses his agility to dodge the Hulk's attacks and tries to figure out a way to break the mind - control. Doctor Strange is called in to use his magic to counter the Joker's mind - games, and the team must fight both the controlled Avengers and the Joker's minions to save the day.
Well, perhaps a powerful and mysterious new threat emerges that neither the Punisher nor the Avengers can handle alone. The Punisher has his street - level combat skills and no - holds - barred attitude, while the Avengers have their superpowers and teamwork. They are forced to team up. During the mission, the Punisher has to deal with the Avengers' more heroic and less lethal approach, which causes some friction at first but eventually they find a middle ground to defeat the enemy.
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.
One idea could be that the Glitchtale characters are transported to the Avengers' universe. Sans could use his shortcuts to surprise the Avengers, and Betty could be a powerful villain they have to face together. Maybe Papyrus befriends Thor because of their similar jovial natures.
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.