One idea could be that Godzilla is drawn to the energy source of one of the Avengers' tech, like Stark's Arc Reactor. This causes chaos as Godzilla approaches the city where the Avengers are based. The Avengers initially see Godzilla as a threat and try to fight it. But then they realize that an even bigger alien force is using Godzilla's presence as a distraction. So, they have to find a way to communicate with Godzilla and work together to defeat the common enemy.
Maybe Godzilla emerges from the ocean during a time when the Avengers are already dealing with a powerful villain on the coast. The massive destruction caused by Godzilla makes the situation even more dire. Hulk could be particularly interested in facing Godzilla, seeing it as a worthy opponent. The other Avengers would have to figure out how to stop Godzilla without causing too much collateral damage and also deal with the original villain. They might discover that Godzilla has a natural instinct to fight the alien - themed villains due to its own history of battling strange creatures from the deep.
In a fanfiction, Godzilla could be a pre - existing but unknown threat that S.H.I.E.L.D. has been secretly studying. When the Avengers get wind of this, they decide to investigate. Thor might think that Godzilla is some sort of primordial beast similar to the creatures in Asgardian legends. Captain America would be concerned about the safety of the civilians. Black Widow and Hawkeye could be sent in first to gather intelligence. Iron Man would start building some sort of device to either communicate with or subdue Godzilla. Eventually, they all come together to face Godzilla and figure out if it's friend or foe.
In an Avengers Godzilla crossover fanfiction, there could be an epic battle. The Avengers might initially be shocked by Godzilla's size and power. Iron Man could try to analyze Godzilla's weaknesses while Thor might see it as a worthy opponent. Hulk could be excited to face such a huge foe. They might have to figure out a way to either stop Godzilla if it's rampaging through a city or team up with it to fight a greater threat.
One idea could be that Godzilla emerges in the world of Pacific Rim during a Kaiju attack. The Jaegers initially see Godzilla as another threat but then realize he's actually fighting the same enemy. They could team up, with Godzilla's raw power and the Jaegers' advanced technology combining to defeat the Kaiju once and for all.
One idea could be that Nightwing is transported to the Avengers' universe through some sort of interdimensional rift. He then has to team up with the Avengers to fight a common enemy, like Thanos who has somehow acquired some of Batman's tech. Nightwing's acrobatic skills and detective abilities would be a great addition to the Avengers' power set.
One idea could be a Godzilla crossover with King Kong. They could start off as enemies but then have to team up to fight a common, greater threat like an alien invasion. This would bring in the classic monster -vs- monster dynamic at first and then shift to an alliance against a new foe.
How about a Godzilla crossover with the Avengers? Maybe in a world where the Avengers exist alongside the Kaiju. Godzilla could be seen as both a threat and an ally. Iron Man could try to study Godzilla's power source while Thor might see him as a kind of primordial force. Captain America would be the one trying to keep everyone in line and figure out how to co - exist with this giant monster.
I think Hulk would be the most interesting. He's all about brute strength and smashing things. Facing Godzilla, he'd be like a tiny but very angry force against a huge monster. It would be a battle of the titans in a sense, and seeing how Hulk reacts to Godzilla's size and power would be really exciting.
One idea could be that Spider - Gwen somehow gets transported to the Avengers' universe. Maybe Doctor Strange's magic goes awry during an experiment. She could team up with Black Widow, sharing her acrobatic skills and they take on a group of villains together. It would be a cool dynamic as Gwen is more of a street - level hero while the Avengers often deal with bigger threats.
Imagine if Elena Gilbert was somehow connected to an Infinity Stone. This would draw the attention of Thanos, but also the Avengers. The Vampire Diaries characters would have to protect Elena while also dealing with the fact that the Avengers are now in their small town. There could be a lot of mistrust at first, but as they learn more about each other's motives and capabilities, they form an alliance to keep the Infinity Stone out of Thanos' hands.
Maybe Zelda herself could be transported to the Avengers universe. With her wisdom and magic, she could assist the Avengers in different ways. She could use her magic to heal the wounded Avengers or help Tony Stark understand some ancient technology that has similarities to her own world's tech. She could also provide strategic advice based on her experience dealing with Ganon in her own realm.
One idea could be that the Avengers are somehow transported to the wizarding world. They might be initially confused by the magic but then start to work with the Hogwarts students and teachers to fight a common enemy, like a dark wizard who has also harnessed some sort of Muggle - like technology to become even more powerful. For example, Voldemort could team up with Hydra remnants to create a super - weapon that threatens both the magical and non - magical worlds.