Well, a cool plot for Sonic Lost World fanfiction could be that Sonic loses his speed temporarily in the Lost World. This forces him to rely on his other skills and wits. He might have to use the environment to his advantage more, like using the strange plants and structures to slow down his enemies. Also, a story where the inhabitants of the Lost World think Sonic is some sort of prophesied hero who will save them from an ancient evil that's starting to awaken. So, Sonic has to decide if he wants to take on this new role while also trying to find a way home.
One idea could be Sonic getting stuck in an alternate dimension within the Lost World and having to team up with some of the local creatures he usually doesn't interact with to find his way back. Another could be a story where Tails discovers a hidden power within the Lost World that could potentially save Sonic's life when he's in a really tough situation against Eggman. Or perhaps a fanfiction where the Lost World starts to merge with Sonic's normal world and he has to figure out how to stop it before chaos ensues.
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 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.
One idea could be a crossover with another Sega character like Bayonetta. Sonic could team up with her to fight against a common enemy that has the power to both slow down time (like Bayonetta's Witch Time) and manipulate the chaos energy in Sonic's world. They would have to combine their unique abilities to defeat this new threat.
One idea could be Sonic having to face a corrupted form of Dark Gaia that has new and unexpected powers. Maybe it can manipulate time in certain areas, causing Sonic to have to race against time in a whole new way.