Obviously, Sonic himself would be likely involved. He might be the one to discover baby Infinite. Tails could also be there, using his intelligence to figure out what happened to Infinite. Knuckles might get involved in protecting the baby Infinite. And of course, there could be other characters like Amy who would be interested in taking care of the baby.
In this fanfiction, perhaps the transformation into a baby for Infinite in Sonic Forces is a result of some sort of magic or experiment gone wrong. The story might then focus on how he interacts with the Sonic universe now that he's in this vulnerable state. He could befriend characters who were previously his enemies, as they see him in a new light. It could also explore how he grows and develops, even in his baby form, and how that might change his future when he eventually returns to his normal state.
Obviously, Aria herself is the main character. Her family members like her mother and father could be important characters, offering support or having different opinions. Her best friends would also be likely to be involved, providing emotional support or getting into some friendly arguments about how she should handle the pregnancy.
Obviously, Sonic himself would be a main character, even in his baby form. His friends like Tails, Knuckles, and Amy would also be main characters. Tails could be the one trying to figure out how to turn Sonic back. Knuckles could be the protector, and Amy could be the caretaker.
Well, there could be her best friend who is always there for her during the pregnancy, giving her advice and emotional support. There might also be a mysterious figure, like an old wise woman who has knowledge about pregnancies in special circumstances that Tera Faya's case might be. And of course, there could be a rival of some sort, who might try to use her pregnancy against her in some way.
Igneel could also play a role. Since Igneel was Natsu's adoptive dragon father, he could appear in Natsu's mind during the transformation, guiding him and giving him advice on how to handle his new dragon powers. And there could be other dragons in the story as well. They might be either friendly or hostile towards Natsu depending on their nature.
There could also be new characters introduced. Maybe other magizoologists from different parts of the wizarding world. They could have different approaches and beliefs about magical creatures. Some might be rivals, trying to outdo Harry in the discovery and study of new species, while others could be mentors, guiding him in the complex world of magizoology.
Obviously Ranma herself. Her father Genma could also be a major character as he has to deal with his daughter's new permanent state. Akane could be there too, and their relationship would likely be very different. Instead of the typical boy - girl rivalry, it could turn into a more complex female - female dynamic.
The Avengers could be involved. Captain America, for instance, might be suspicious of this 'Loki' (who is actually Harry) and keep a close eye on him. Iron Man could use his technology to analyze Harry, thinking he is Loki. And from the Harry Potter universe, Ron could be there, trying to support Harry in this strange situation where he is mistaken for someone else entirely.
Definitely the Naruko clone herself. Then there could be Sasuke. He might be intrigued by the clone and try to understand her existence. Also, Kakashi could be involved, maybe as a mentor figure to the clone, guiding her. As for the 'lemon' side, there could be some random attractive ninja from another village who catches the clone's eye and they develop a romantic relationship.
Obviously, Elliot Stabler could be involved. He and Olivia have a long - standing relationship in the show, and he could be a source of support or conflict depending on the story. Fin Tutuola could also be there, as he's her colleague and friend, and might offer some comical or heartwarming moments as he tries to help her out. Captain Cragen could be in the story as well, dealing with how Olivia's pregnancy affects the work environment.