Maybe in the 'sonic gets fat fanfiction', Sonic discovers a new type of power - up that gives him super strength but as a side effect, it makes him gain a lot of fat. He then has to learn to control this new power without sacrificing his speed and agility. He might go on a journey to find a way to reverse the fat gain while using the power for good.
Dr. Eggman would be a likely main villain. He might be the one who devises a plan to make Sonic fat, thinking that it will slow him down and make it easier for him to be defeated.
Maybe Sonic's friends will take care of him. They could bring him medicine and make sure he rests. For example, Tails might use his inventions to find a quick cure.
It could be about a character named Anerica who is bullied or taunted for being fat, and then she goes on a journey of self - discovery and self - acceptance.
In this kind of fanfiction, it might be that Amy thinks Sonic has been hiding secrets from her. These secrets could be something she believes are harmful to their friends or the world they live in. So she decides to betray Sonic in order to protect everyone else. Sonic would then be left confused and hurt, having to figure out how to fix the situation.
One possible plot could be that Sonic catches a strange virus that slows down his super speed. His friends like Tails and Knuckles would then have to find a cure while protecting Sonic from his enemies who might see this as an opportunity to attack.
In 'sonic gets experimented on fanfiction', one possibility is that the experimenters are from an alien race. They might be curious about Sonic's abilities which are far beyond what they've seen in their own world. So they capture him and start running all sorts of tests on him, from physical examinations to tests on his cognitive abilities.
In such a fanfiction, Sonic might face challenges like his friends being deceived by the imposter. His reputation could be at stake as the imposter might do bad things in his name.
Well, Sonic might be held in a high - tech prison. His friends like Tails would start a rescue mission. They could use their unique skills, Tails with his flying and mechanical abilities, to track Sonic down.
It could be some of the lesser - known badniks that team up to bully Sonic. Since they can't defeat him on their own, they figure that by ganging up on him and bullying him, they can at least make his life difficult.
One possibility is Dr. Eggman. He has always been after Sonic and wants to stop him. Kidnapping Sonic would be a way for him to achieve his evil goals.