Family is also a likely theme. We could see how his family influenced him as a baby. Maybe his parents taught him some basic thieving techniques or passed on their sense of justice, which is an important part of Sly's character in the original games. In these fanfictions, his family could be depicted in different ways, like his father being a strict but loving teacher or his mother being the one who calms him down when he gets into trouble.
There might be a fanfiction where baby Sly is being taken care of by the other members of his gang. They could be teaching him basic skills like climbing or hiding in a very endearing and humorous fashion. For example, Bentley could be trying to teach baby Sly how to read maps, but Sly is more interested in chasing a butterfly.
The human boy might be a lost traveler who accidentally enters the world of Sly Cooper. He is scared at first but quickly adapts. Sly sees potential in him and decides to train him in the ways of thievery. The boy has a natural talent for stealth, which he developed while trying to survive in the wild on his own. With his help, Sly's gang is able to pull off a heist that they previously thought was impossible, stealing a valuable artifact from a heavily guarded fortress.
A fanfiction of Sly Cooper and Carmelita getting married could be full of twists and turns. For example, just as they are about to say their vows, one of Sly's old foes could show up with a plan to disrupt the wedding. But Sly and Carmelita, being the strong characters they are, team up to defeat the foe and then continue with their wedding. Their relationship has always been complex, with Carmelita chasing Sly in the past. Now, as they get married, it symbolizes a new chapter where they are on the same side, facing the future together, and their love overcomes all the past differences and challenges.
One popular 'baby sly cooper fanfiction' might be those that focus on his early training days. It could show how he learned his sneaking skills as a baby in a really cute way, like trying to steal a cookie from the kitchen without being noticed by his family.
First, think about the character of baby Sly. What would he be like? Is he already showing signs of his future thieving skills? Then, create a simple plot. Maybe it's about a day in the life of baby Sly at the Cooper hideout.
One common theme is the struggle between their two worlds. Sly being a thief and Carmelita being a law enforcer. Their love has to overcome this huge obstacle.
One idea for a Sly Cooper fanfiction involving a human girl could be that she is a skilled thief herself, perhaps from a different part of the world. She crosses paths with Sly and his gang during one of their heists. At first, they are suspicious of her, but she proves her worth by helping them outwit the guards in a highly secured museum. As they continue to work together, Sly starts to develop feelings for her unique charm and courage.
One idea for a Sly Cooper fanfiction involving a human boy could be that the boy is a huge fan of Sly Cooper's adventures. He somehow stumbles upon a relic from one of Sly's heists. This relic has some sort of magical power that transports him into Sly's world. There, he has to learn how to survive among the various animal characters and, in the process, he becomes an unexpected ally to Sly and his gang, helping them outwit the bad guys in new and creative ways.
Archive of Our Own (AO3) is also a great place to look. There are many different fanfictions there, and you can filter the search results to find specifically female Sly Cooper fanfictions. It has a large and active community of writers and readers, so you're likely to find some hidden gems.