Well, it could be that a new villain emerges during a period of peace in the Sonic world. This villain starts a war by spreading false rumors and creating unrest. Sonic, Tails, and Knuckles have to work together to uncover the truth behind the villain's actions. They also have to convince the different groups in the Sonic world that they have been deceived, and then join forces to defeat the real enemy and restore peace.
Perhaps in this fanfiction, Sonic and his allies could discover an ancient artifact during a time of war. This artifact has the power to either end all wars or cause total destruction. They then embark on a journey to figure out how to use it for peace, while being chased by various factions who want to use it for their own gain. This would involve a lot of action sequences as Sonic tries to outrun and outwit his pursuers.
In a fanfiction plot, Natasha could become a healer on the war front where Andrei is fighting. She disguises herself as a boy to be there. Andrei gets injured and she takes care of him. This brings them closer in a different way than in the original story. They then have to deal with the secrecy of her identity and the challenges of war while falling deeper in love.
A possible plot for 'sonic meets sonic boom fanfiction' is that a strange rift in the dimensions brings Sonic and his friends from the classic Sonic world into the Sonic Boom world right when they are about to have a big celebration. However, the Sonic Boom characters are initially suspicious of these new arrivals. But as they start to realize that a common enemy is behind the rift, they join forces. Sonic and Sonic Boom Sonic could have a friendly competition to see who can defeat the most badniks during the adventure, which also helps them bond and understand each other's strengths better.
One plot idea could be Dark Sonic having to face a new villain that is even more powerful and is threatening to destroy his world. He has to find a way to control his dark powers even more to defeat this foe.
One idea could be Sonic discovers a magical land where all the characters are in an abdl state. He has to navigate this new world while also dealing with his own changed circumstances. For example, he might have to learn to control his new abilities that come with this state.
One idea could be Naruto using his unique ability to communicate with all living things to broker peace between feuding clans. He might use his influence over animals to show the clans how interconnected all life is.
One idea could be that Sonic from Sonic X accidentally travels to the Sonic Boom universe and has to adapt to the different environment and new versions of his friends. Maybe he teams up with Sonic Boom Sonic to defeat a common enemy that has found a way to cross between the two universes.
In some fanfictions, they might explore the differences in their personalities. For example, Classic Sonic's more simplistic and straightforward approach contrasted with Modern Sonic's more complex and sometimes cocky attitude. They could start off not getting along due to these differences but then learn to work together. Maybe there's a story where they are both trapped in a strange dimension and have to rely on each other's skills. Classic Sonic's spin - dash and ability to handle more 2D - like terrains, while Modern Sonic can use his homing attack and boost in the more complex 3D areas of the dimension.
One idea could be a crossover adventure where Sonic and the Mane Six team up to stop a powerful villain that threatens both their worlds. They could combine their unique abilities, like Sonic's speed and the magic of the Mane Six.
One idea could be a new character being introduced into the Sonic Black Knight world. This character could be from another dimension and have unique powers. They could initially be seen as a threat but end up teaming with Sonic to fight a greater evil. Another plot could be Sonic losing his sword skills and having to go through a series of challenges to regain them, all while facing off against old enemies with new twists.
A plot could involve Eggman creating a device that accidentally swaps their minds. Sonic now has to figure out how to use Eggman's technology to reverse the process while in Eggman's body, and Eggman, in Sonic's body, is forced to experience what it's like to be the hero for a while. This would lead to some comical and also thought - provoking situations as they both learn about the other's perspective.