Well, Sonic himself is often really popular in fanfics. He's the main character of the franchise after all. His speed and adventurous spirit make him a great centerpiece for many stories. Writers can easily create exciting plots around his adventures, battles against Eggman, and his relationships with other characters.
One common theme is friendship. Sonic, Tails, Knuckles, and the other characters have such strong bonds, and fanfics often explore these relationships further. For example, how they support each other during tough battles or when facing personal problems.
Knuckles could also make an appearance in Sonic the Merhog fanfics. He might be guarding some underwater treasure or ancient relic. Sonic has to convince Knuckles that they need to work together to protect it from a common enemy, like a group of pirates who are trying to steal the underwater treasure.
Sonic himself is a must - include. His speed and adventurous spirit drive the story forward. Tails is also great as he can provide technical support and is Sonic's loyal friend. And Dr. Eggman, as the main antagonist, always adds conflict and excitement to the story.
Fiona Lemon is another main character. She might have her own unique set of skills or traits that complement Sonic's. She could be intelligent, brave, or have some special power that helps in their adventures together.
Well, obviously the main characters would be the Powerpuff Girls - Blossom, Bubbles, and Buttercup. And Sonic the Hedgehog would also be a central character. There could also be other characters from both universes like Professor Utonium from the Powerpuff Girls side and Tails or Knuckles from Sonic's side depending on the direction of the fanfic.
There might also be new characters created specifically for the fanfic. These could be hybrids of the two universes or characters that are designed to bridge the gap between Shrek's world and Sonic's world. They could play important roles in the plot, like being the ones who bring Shrek and Sonic together in the first place or being the key to solving the problems that arise during their adventure.
Sonic himself is always popular. His speed and heroic nature are central to many fanfics. Then there's Sally. She is often a key character, especially in stories that involve strategy and leadership. Bunnie is also well - liked. Her mechanical enhancements can add an interesting element to the story. And of course, Antoine, with his comical French accent and sometimes clumsy but brave nature, makes for great character interactions in the fanfic world.
In a Metal Sonic x Sonic Fanfic, the action and adventure aspects can be really exciting. Writers may create new challenges for Sonic and Metal Sonic to face together or against each other. For example, they could be forced to team up to defeat a greater evil that threatens their world. This kind of fanfic also gives a chance to delve into the emotions of the characters. Metal Sonic, although a machine, might show signs of evolving emotions or at least a form of artificial intelligence - based loyalty or rivalry towards Sonic.
A 'sonic x sonic unleashed fanfic' could be really interesting. It might explore new adventures for Sonic in the context of the Sonic Unleashed world. Maybe it shows Sonic facing new challenges or forming new relationships within that setting.