One possible plot could be that the Avengers are investigating a series of strange disappearances in a small town. They soon discover that vampires from The Vampire Diaries' world are behind it. There could be a face - off between Thor's hammer and the super - strength of some of the older vampires. Maybe Damon could team up with Iron Man, with their shared sarcastic sense of humor, to outwit the main villain who is causing chaos in both their worlds.
A plot could revolve around a spell gone wrong. A witch in The Vampire Diaries tries to create a powerful protection spell but instead opens a portal that sucks in the Avengers. The Avengers are confused at first but quickly adapt. They learn about the vampires and their hierarchy. Loki, being Loki, might try to manipulate some of the vampires to his own advantage. But then the Avengers and the good Vampire Diaries characters like Stefan realize Loki's plan and have to work together to stop him and also close the portal that's causing all kinds of supernatural disruptions in both universes.
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.
It could be really exciting. Maybe Damon Salvatore from Vampire Diaries teams up with Iron Man. Damon's charm and his vampire skills combined with Iron Man's high - tech gadgets would make for some epic battles against common enemies. They could be fighting against a new breed of super - vampires created by Hydra or something like that.
Another great pairing could be Caroline Forbes and Black Widow. Caroline is a strong - willed vampire who has grown a lot throughout The Vampire Diaries. Black Widow is a highly skilled spy and fighter. They could bond over their shared experiences of having to be tough in male - dominated worlds. Maybe they would train together, with Caroline teaching Black Widow some vampire - related combat skills and Black Widow showing Caroline some new spy techniques.
The clash of powers. Naruto's chakra - based jutsu against the vampires' superhuman abilities like speed and compulsion. It makes for some exciting battle scenes.
Well, perhaps Hermione is sent to the 'Vampire Diaries' world to retrieve a powerful magical artifact that has somehow ended up there. Along the way, she meets Damon and Stefan. She forms an unlikely friendship with them as they help her search for the artifact. In return, she uses her magic to protect them from some of the more dangerous supernatural threats in their world.
A really interesting plot twist is when a character from Arrow gets turned into a vampire in some fanfictions. For example, in 'The Dark Transformation', Felicity Smoak gets bitten by a vampire and has to deal with her newfound vampiric nature while still trying to help Team Arrow. It completely changes her character dynamics and adds a whole new layer of complexity to the story.
One idea could be that some of the Glee characters visit Mystic Falls for a singing competition. While there, they get caught up in the vampire drama. Maybe a vampire takes a liking to Rachel's voice and wants to turn her into a vampire so she can sing forever. Another plot could involve the vampires using the Glee club as a cover to hunt during their events.
If we consider the Hulk, he would be a very intimidating presence to the vampires. But the vampires, especially someone like Klaus Mikaelson, wouldn't be easily intimidated. Klaus might see the Hulk as a worthy opponent or a potential tool in his power - hungry schemes. On the other hand, Natasha Romanoff could bond with Caroline Forbes over their strong female personalities. They could share fighting techniques and strategies. And Thor would be quite fascinated by the different supernatural elements in Mystic Falls, seeing it as another realm of mystery similar to Asgard.
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.