In addition to Sally, there could be other robotic characters in the fanfiction. Maybe there is a group of robots that Sally meets after being roboticized. They could either be friendly and help her understand her new state, or be hostile and see her as an outsider. And of course, her family members could also be characters, dealing with the changes in Sally.
One possible plot could be that Sally is roboticized against her will. She then has to struggle to retain her humanity while dealing with the new abilities and limitations of her robotic form. Maybe she tries to find a way to reverse the process.
Well, in this 'sonic gets roboticized fanfiction', it might explore the idea of Sonic's identity when he becomes robotic. It could show how his friends react to this new form. For example, Tails might be shocked at first but then try to find a way to reverse the roboticization. And Sonic himself could be struggling with feeling less 'alive' in his robotic state, all while facing new enemies or challenges that are related to his new robotic nature.
Fans of Sonic who have a creative streak and an interest in exploring different scenarios for the character might write it. They could be young writers who are really into the Sonic universe and want to put their own spin on things.
One possible plot could be that Tails is captured by an evil scientist. The scientist then roboticizes him against his will. Tails has to struggle to keep his original self and memories while being forced to do the scientist's bidding. Eventually, he might break free from the robotic control through the power of friendship with Sonic and the others.
One possible plotline could be Sonic struggling to come to terms with his new robotic parts. He might face challenges in his speed or his usual acrobatic abilities and have to relearn how to move and fight. For example, his robotic parts could malfunction at crucial moments.
A rogue robotic entity could be the main antagonist. This entity has the ability to roboticize other beings and views Tails as a prime target. It wants to use Tails' body as a vessel to spread its robotic influence further, so it sets out to roboticize him and then use him as a tool for its own dark purposes.
Sorry, I don't have a specific 'sonic gets partially roboticized fanfiction' to recommend directly. But you can try searching on some popular fanfiction websites like Archive of Our Own or FanFiction.net.
The main characters are clearly Percy and Sally. Percy is likely a central figure who will have to deal with the consequences of getting Sally pregnant. Sally, on the other hand, is the one who is going to have the baby, so she will also be facing a lot of physical and emotional changes.
Another aspect could be the challenges they face during the pregnancy. Sally might have health issues, and Percy would have to step up and take care of her. He could start working extra jobs to save for the baby's future. Also, they might need to find a new place to live that is more suitable for a family.