Well, aside from Shadow, Omega is a really cool character to include. His robotic nature and his loyalty to Shadow can create some interesting storylines. Tails could also be included, especially if you want to explore the scientific side of the Sonic universe in your fanfiction. He can interact with Shadow in various ways, like helping him with some sort of technological problem or even getting into an argument about the ethics of certain actions. Another popular character is Knuckles. His guardian role of the Master Emerald can bring in elements of mystery and adventure when combined with Shadow's story.
Sonic is a very popular character to pair with Shadow. Their rivalry and yet potential for teamwork makes for great fanfiction. Tails is also often paired with Shadow. Tails' intelligence can complement Shadow's combat skills. And Knuckles is another one. His strength and connection to the Chaos Emeralds can create interesting storylines when paired with Shadow.
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.
Knuckles would be a great addition. His strength and his role as the guardian of the Master Emerald can add a lot of depth to the story. He could have some interesting interactions with Coldsteel, perhaps even getting into a friendly rivalry. Also, Shadow could be included. His dark and brooding nature would make for an interesting dynamic with Coldsteel.
Well, aside from the original team, Janine Melnitz can be a great addition to Ghostbusters game fanfiction. She has a sassy personality and can bring a different dynamic to the story. Also, Dana Barrett is often included, especially if you want to add a bit of a romantic subplot. And of course, the Ghostbusters themselves are always popular. You can explore their relationships, like the friendship between Ray and Egon, or Peter's witty banter with Winston.
Ben Tennyson is obviously a must - as he is the main character. Gwen is also very popular as she has her own powers and often helps Ben. Kevin Levin is another great choice. His ability to absorb materials and his complex relationship with Ben add a lot of depth to the story. Vilgax is a classic villain that makes any fanfiction more exciting. He's powerful and has a long - standing rivalry with Ben.
Fluttershy could be an interesting character to include. Her gentle nature could be a contrast to Shadow's more brooding personality. It could lead to situations where Shadow learns to be more empathetic or Fluttershy becomes more assertive in his presence. Sunset Shimmer is also a key character as she has a complex backstory and her knowledge of both worlds can play a significant role in a fanfiction involving Shadow.
Percy himself is a must - include. His adventures and his powers as a son of Poseidon always make for great fanfiction material. Also, Annabeth. Her intelligence and her relationship with Percy add a lot of depth.
Misfortune herself is obviously a very popular character to include. Her mother is also often written about as she plays a big role in Misfortune's life. And then there's Mr. Voice, that mysterious entity that guides Misfortune. Writers like to explore his true nature in fanfiction.
The mysterious old hermit who lives deep in the woods can also be a great character to include. There are so many things that can be done with this character. Maybe they know the secrets of the woods and can either help or hinder the other characters depending on their motives.
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.