A possible plot is that a strange force is corrupting the Freedom Fighters, and Sonic has to find a way to save them and also figure out what's causing this corruption. This could involve a lot of action as Sonic races against time to free his friends and stop the source of the evil.
There could also be other characters depending on the specific fanfiction. Some might include Dr. Eggman as an antagonist, always trying to cause trouble for Sonic and the Freedom Fighters, or other minor characters that add to the story's complexity and depth.
A possible plot point could be a mystery that both Sonic Heroes and the Freedom Fighters need to solve. It could involve a powerful and ancient artifact that has been lost for ages. The two groups, with their different abilities and perspectives, would have to work together to figure out where the artifact is and how to use it to save their world. There could be many twists and turns in this search, like false leads and unexpected allies.
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 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.
One idea could be that Sonic and Mario have to team up to save both their worlds from a new, powerful villain who has the ability to manipulate time. This would force them to use their unique speed and jumping abilities in new and creative ways.
One idea could be a crossover where Sonic meets characters from other popular franchises in a fanfiction. For example, Sonic could meet Mario and they have to team up to save both their worlds from a common threat. Another idea is a time - travel fanfiction where Sonic goes back in time and meets his younger self, leading to some self - discovery and adventures. Also, a fanfiction about Sonic meeting a powerful new villain created by a fan could be really exciting.
One idea could be Sonic's speed causing chaos among the Justice League at first. For example, he might accidentally disrupt Batman's plans during a Gotham operation. Another could be Sonic teaming up with The Flash to outrun a common enemy that is threatening both their worlds. Maybe they have a friendly race while also trying to lead the bad guy into a trap.
One idea could be that Sonic Underground's siblings get separated in a new zone and Shadow accidentally stumbles upon them. He initially thinks they are enemies but then realizes they are on the same side against a common foe. Another could be that Shadow is on a mission related to the Chaos Emeralds and crosses paths with Sonic Underground. They end up having to team up to stop a new villain who wants to use the power of music and the Chaos Emeralds for evil.
One idea could be that Bendy and Sonic find themselves in a strange, in - between world where the art styles of their respective universes start to blend. They have to team up to find a way back to their own worlds. Maybe there are some glitches in this new world that they need to fix together.
One interesting idea could be the clash of their different power systems. Goku has his ki-based attacks and Super Saiyan forms, while Sonic has his speed and Chaos Emerald powers. Maybe they could have a friendly sparring match to test each other's abilities.