Perhaps Tails gets tricked by an evil villain who convinces him that Sonic and their friends have been using him all this time. Feeling betrayed, he turns evil and teams up with the villain to take revenge. He could then use his mechanical skills to create dangerous weapons for their evil schemes.
It could be that Tails suffers some kind of great loss that warps his mind. For example, if he accidentally causes harm to someone he loves due to one of his inventions going wrong. This guilt and trauma might lead him to turn evil as he blames the world and his friends for not being able to prevent it. He then sets out to 'fix' the world in his own, dark way.
One possible plot could be that Tails is exposed to some dark energy or a powerful, corrupting artifact. This makes him gradually turn against his friends. He might start using his intelligence to outwit Sonic and the others for his new, evil goals.
One possible plot could be that Tails is kidnapped by an old enemy seeking revenge. Maybe Eggman wants to use Tails' intelligence for his own evil schemes. He could hold Tails captive in his secret lair and try to force him to build some powerful weapon.
One possible plot could be that Tails is betrayed by a close friend who becomes jealous of his intelligence and skills. For example, Sonic might start to feel overshadowed by Tails and in a moment of weakness, turn against him. This could lead to Tails having to fend for himself and prove his worth all over again.
Another plot could involve Tails trying to hide his sickness at first because he doesn't want to worry his friends. But as it gets worse, he can't hide it anymore. His friends then feel guilty for not noticing earlier and they all come together to take care of him and find a remedy.
Dr. Eggman could still be the main antagonist. Even if Tails turns evil, Eggman might see him as a threat to his own evil plans. He could try to stop Tails while also taking advantage of the chaos Tails is causing among Sonic and his friends.
In a 'Tails Turns into a Robot' fanfiction, it could be that an ancient technology is accidentally activated by Tails. This technology fuses with him and turns him into a robot. He then discovers that he has a special connection to an ancient robotic civilization. He has to navigate his new identity, learn about this civilization, and decide whether to embrace his robotic self or try to find a way back to his normal form all while facing challenges from those who fear or want to use his new - found powers.
Another plot could involve Naruto being influenced by a powerful and evil entity. This entity could be a new, powerful villain who manages to corrupt Naruto's mind. It could start with small suggestions that gradually change Naruto's perspective. As he becomes more and more influenced, his actions start to become more self - centered and less about protecting the village and more about achieving his own, perhaps twisted, goals.
Maybe the mansion is a place where time and space are distorted. Luigi and Tails get separated at first. Luigi has to find Tails while dealing with the strange things in the mansion like rooms that change shape. Tails, on the other hand, is trying to find a way to stabilize the time - space distortion. Eventually, they find each other and combine their efforts to fix the situation.
One possible plotline could be that Harry discovers a dark secret about his family and feels betrayed by the wizarding world. This could lead him to side with the dark forces. Another could be that he is influenced by a powerful dark wizard who manipulates his mind. Maybe he becomes envious of the power others hold and decides to take it by any means, turning evil in the process.
A possible plot could be that Mariep Dean's transformation is temporary. While she is a girl, she experiences things like first love or girl - only events. But she is desperate to turn back to her original self, so she goes on a quest to find a way to reverse the change.
One possible plotline could be that Naruto is constantly shunned and mistreated by the village despite his efforts to protect it. This continuous rejection could gradually turn his heart cold and make him seek revenge. He might start using his powerful jutsu against those who once wronged him, like attacking the Hidden Leaf's leadership.