One common theme is adventure. Since Sonic, Rouge, and Knuckles are all characters associated with action and exploration, many fanfictions focus on them going on quests or facing new challenges. For example, they might be looking for a lost relic or trying to stop an evil force from taking over their world.
A 'Metal Sonic x Shadow Fanfic' might explore an interesting dynamic between the two characters. It could involve their unique abilities interacting in different scenarios. For example, Metal Sonic's robotic precision and speed combined with Shadow's chaos powers could lead to some really intense action scenes. Also, the fanfic might delve into their personalities, perhaps showing a rivalry or an unexpected alliance.
The fanfic could be centered around a powerful Halloween artifact that is said to grant great power. Rouge wants it for her collection and power - up, Knuckles wants to protect it as it might be dangerous in the wrong hands, and Shadow wants to study it. This leads to a complex situation where they initially fight over it. However, as they start to understand the true nature of the artifact and the real threat behind it (maybe an ancient evil that awakens on Halloween), they join forces to deal with it and prevent disaster.
One idea could be a story where Knuckles and Rouge have to team up to protect a hidden ancient relic from a new villain in the Sonic universe. They start off distrusting each other as usual, but as they face challenges, they gradually learn to work together better.
Well, 'The Mysterious Power' is another great one. It begins with Knuckles noticing a strange power around the Master Emerald. Rouge, being curious, decides to investigate. Sonic, as always, is there to help. The story unfolds with lots of mystery and action as they try to figure out the origin of this power and how to stop it from causing chaos.
One plot idea could be that on Halloween, Rouge discovers an ancient artifact in a spooky, abandoned castle. She enlists Shadow and Knuckles to help her figure out what it is. As they explore the castle, they encounter various traps and spooky creatures. Along the way, they start to have some friendly competitions and arguments, but also grow closer as a team.
For Rouge, emphasize her cunning nature. In a Halloween setting, she could be using her skills to find hidden treasures or secrets. Describe her sly smiles and quick thinking. For Shadow, show his brooding side but also his loyalty. Maybe he's reluctant to get involved in Halloween hijinks at first but does it for his friends. For Knuckles, play up his strength and protectiveness. He might be on high alert during Halloween, thinking there are threats to his island.