Well, Shadow is a really popular character to include. His dark past and his similarities and differences with Sonic can create a lot of interesting storylines. Rouge the Bat is also a great choice. Her thieving ways and her complex relationships with other characters can bring a different flavor to the fanfiction. And don't forget about Dr. Eggman. He's the main villain in Sonic X, and his wacky inventions and evil plans are essential for creating conflict in the story.
Well, you could focus on Sonic's origin story in the Sonic X Rewritten fanfiction. Maybe there was a long - lost civilization that was responsible for his super speed. Or you could have a plot where Sonic loses his speed for a while and has to rely on his other skills and the help of his friends to get it back. And a time - travel plot would be cool too. Sonic and his friends could go back in time to prevent a disaster that would change the future of their world.
Amy is another character that shows up a lot. Writers might explore her relationship with Sonic more deeply, like how she supports him during the rebuilding process or if she has any new dreams or goals in the post - adventure world.
Definitely Sonic. He's the star of the show. Then there's Amy. She can bring in the element of love and determination. Shadow could also be a great addition. His brooding and powerful nature can create some interesting storylines, especially if he has to team up with Sonic in the Lost World. And don't forget about Cream and Cheese. Their cute and innocent presence can contrast with the more dangerous aspects of the Lost World.
In sonic the hedgehog crossover fanfiction, Sonic himself is of course the main character. But other characters that are popular to include are Amy Rose. Her love for Sonic can create interesting relationship dynamics in crossovers. Another popular one is Dr. Eggman. His villainous nature can be a great source of conflict in crossovers with other characters. And then there are characters from other video game series that are also quite popular, like Link from The Legend of Zelda. His use of weapons and exploration skills could make for some really exciting crossovers with Sonic.
Sonic, without a doubt, is the most crucial character. But in addition to him, characters like Silver can bring a different perspective. His ability to manipulate time in a way could be incorporated into the Black Knight story in unique ways. Also, Rouge. Her thieving skills and her relationships with other characters can be a great addition. And don't forget about the characters native to the Black Knight world, like King Arthur - like figures, who can add a touch of medieval fantasy to the fanfiction.
Sonic himself is usually really popular. Whether it's the Sonic from Sonic X or Sonic Underground, he's the main hero and often at the center of the action. People love reading about his adventures and how he deals with the challenges in the combined universes.
Classic Sonic and Modern Sonic are the stars, of course. However, Knuckles can also be a very popular character in these fanfictions. His role as the guardian of the Master Emerald in the classic world and his continued importance in the modern Sonic universe make him an interesting addition. He can be the one who initially doubts the partnership between the two Sonics but then comes around. And Amy Rose is another character that often appears. Her feelings for Sonic, both Classic and Modern, can add a touch of romance or at least some emotional depth to the story.
Tails is a very popular character to include. He is Sonic's best friend and his technical skills can add a lot to the story. For example, he could help Sonic and Shadow figure out some complex machinery or technology that is part of the plot.
Mario and Sonic themselves are the most obvious and popular choices. They are the main characters of their respective franchises and their rivalry is always exciting to explore.
Princess Peach from the Mario universe is often included. She can add a touch of elegance and sometimes be a damsel in distress or a helpful ally. Tails from Sonic's world is also popular. His intelligence and flying ability can be useful in various situations in the fanfiction, like helping Sonic and Mario solve puzzles or reach high places during the Olympic events.