Well, for 'Avengers as animals fanfiction', Hulk could be a huge and indomitable grizzly bear. His raw strength and temper would match that of a bear. Black Widow could be a sleek and stealthy panther, able to move quietly and strike quickly. Hawkeye could be an eagle with excellent vision for his archery skills.
One idea could be exploring what would happen if the spirit animals had the ability to communicate with other species in the wild. For example, how would a wolf spirit animal interact with a group of deer? Another idea is to create a fanfiction where the bond between the human and the spirit animal is tested in extreme situations, like a famine or a natural disaster. You could also write about a new character who discovers an ancient and powerful spirit animal that has been lost for centuries.
One idea could be creating an OC who has a unique power related to time manipulation. They could be from a different dimension and end up in the Avengers' world. Another idea is an OC who is a master of ancient magic and teams up with Doctor Strange within the Avengers group. Maybe an OC that is a super - intelligent android created by a villain but has a change of heart and joins the Avengers.
One idea could be an Avengers crossover with the Harry Potter universe. The Avengers could visit Hogwarts and interact with the wizards and witches. Maybe Iron Man tries to understand the magic with his scientific mind. Another idea is a crossover with the Star Wars galaxy. The Avengers could team up with the Jedi or the Rebels to fight a common enemy. For example, Thor could have an epic battle alongside a Jedi Knight.
In a 'naruto avengers fanfiction', there could be a story where the Avengers are sent to the Naruto world. They have to adapt to the ninja way of life. Thor could learn about chakra and how it's different from his Asgardian powers. Black Widow could study the ninja arts of deception and stealth from the likes of Kakashi. And Hulk could have an interesting interaction with the tailed beasts, perhaps even learning to control his anger in a new way from Naruto's experiences with Kurama.
A great idea for avengers community fanfiction is to have a story where the Avengers are faced with a threat that is specifically targeting their weaknesses. For instance, a villain who can manipulate technology goes after Iron Man, while another who can control minds targets Scarlet Witch. The Avengers then have to figure out how to protect each other and defeat this new enemy together.
One idea could be exploring new heroes joining the Avengers. Maybe a young hero from a different dimension discovers the Avengers and tries to prove themselves worthy of joining. Another could be a story where the next generation of Avengers has to face a threat that was created by a past Avenger's mistake. For example, an experiment gone wrong that now threatens the world.
One interesting idea could be the superpowers the babies inherit. For example, a baby of Iron Man might show an early aptitude for technology or engineering. Another could be how the other Avengers interact with the babies, like Thor trying to teach them about Asgardian culture in a really fun and comical way. And also, there could be a plot where the babies are in some sort of danger and the Avengers have to use their unique skills to protect them.
A fun idea could be a crossover where Gohan's friends from Dragon Ball also get involved. The Avengers are initially confused by these strange new allies with tails. Gohan has to act as a bridge between the two groups, and together they face a common enemy that requires both the technology of the Avengers and the martial arts skills of Gohan and his friends.
One idea could be to explore a new villain that has powers related to time manipulation. The Avengers would have to figure out how to combat this villain who is constantly changing the past and future to gain an advantage. Another idea is to focus on a character development arc for a lesser - known Avenger, like Hawkeye, where he has to overcome a personal fear or struggle while on a mission.
A really cool plot idea could be that the Avengers are watching Endgame in a sort of 'alternate reality cinema' where the events in the movie start to bleed into their real world. So they have to stop the movie from fully playing out to prevent chaos in their own universe. This would involve a lot of action and strategic thinking as they try to stop things like Thanos from emerging from the screen while also trying to understand the significance of what they're seeing in the movie.