A great avengers and batman fanfiction could involve Thor and Batman. Maybe Thor's hammer ends up in Gotham and Batman, being the detective he is, tries to figure out how to return it to Thor while also protecting it from villains who want to use its power for their own evil purposes.
The crossover where Batman takes on the role of a strategist for the Avengers during a major battle against an alien invasion. His tactical mind helps the Avengers in a way they've never experienced before.
One idea could be Batman using his detective skills to uncover a hidden threat that the Avengers initially overlook. For example, he might notice some small, seemingly insignificant details during a battle scene that lead him to a much larger conspiracy.
A great plot could be that Batman, being the master detective, discovers a plot by some unknown force that is targeting both Gotham and the Avengers' main cities. He contacts the Avengers for help. They initially have some trust issues as Batman is a loner and the Avengers are used to working as a team. But as they start to understand each other's methods, they combine forces. For example, Batman's stealth and strategic skills combined with Captain America's leadership and combat skills could make for some really cool fight scenes against the common enemy.
In some Batman vs Avengers fanfiction, a great plot twist is when Batman manages to hack into the Avengers' communication system and makes them fight among themselves. He then uses the chaos to his advantage. Another cool twist is that there's a greater threat looming, and Batman and the Avengers have to put aside their differences and work together. But as they do, they discover that one of the Avengers has been replaced by a shape - shifter, and Batman has to figure out who it is before it's too late. There could also be a plot twist where Batman uses a device that actually amplifies his own strength and skills to the point where he can match the Avengers' superpowers, at least temporarily.
Batman would be the most interesting. He doesn't have superpowers like the Avengers, but he's still a formidable hero. His presence in their world would be like a wild card. He'd be constantly analyzing the Avengers and their enemies, and his dark, brooding nature would contrast with the more flashy and super - powered Avengers.
Batman and Black Widow would be really interesting. Batman is all about mystery and detective work in the shadows, while Black Widow is a master spy. They could learn a lot from each other's skills in stealth and intelligence gathering. Also, Thor and Batman could be a great focus. Thor with his god - like powers and Batman with his human ingenuity. Imagine Batman trying to figure out the science behind Thor's hammer while Thor is amused by Batman's determination in the face of the supernatural.
One idea could be that Batman, in his quest to fight crime in Gotham, discovers some sort of interdimensional rift that leads him to the world of The Avengers. He then has to team up with them to stop a common enemy that is using technology from both universes to cause chaos. For example, Loki could be teaming up with some of Batman's rogues gallery like The Riddler to create complex puzzles and illusions that are both magical and technological in nature.
Batman would fit in by being the ultimate strategist. The Avengers sometimes jump into action without a fully thought - out plan. Batman's meticulous planning would force them to be more organized. He could also provide a more grounded perspective. Many of the Avengers have superpowers and deal with large - scale threats from outer space or magic realms. Batman, coming from the streets of Gotham, would remind them of the importance of protecting the everyday people.
One interesting story could be about a new member joining the Mighty Avengers in the fanfiction world. This new character might bring a unique power that helps the team in unexpected ways, like being able to manipulate time on a small scale to reverse some of the damage caused by villains.
Some avengers cartoon fanfiction focuses on the personal relationships within the team. There could be a story about Thor and Loki's relationship evolving in a different way. Maybe Loki is trying to truly reform and be a part of the Avengers, but the other members are skeptical. This type of fanfiction delves deep into the characters' emotions and how they interact with each other on a more personal level rather than just fighting villains.