The plot could be centered around the chaos that ensues when Itachi turns into a baby. His enemies might see this as an opportunity to attack, thinking he's vulnerable. However, his allies would protect him. And as they take care of him, they might discover new things about Itachi's past or even his true nature that was hidden before. There could be many comical scenes like when they try to diaper him or feed him baby food and he makes a fuss.
Obviously, Itachi as a baby would be a main character. Sasuke would also be a key character as Itachi's brother. Their relationship would be very different now that Itachi is a baby. Naruto could also be involved, perhaps being curious about this strange transformation and wanting to help out in some way.
One idea could be that in this fanfiction, Wolverine turns into a baby due to a powerful mutant's accidental spell. The X - Men then have to take care of him while also trying to find a way to reverse the transformation. They might discover new aspects of Wolverine's personality as a baby, like his unwavering determination is now shown through his attempts to crawl towards danger.
Well, it could start with a simple moment of curiosity. Naruto, being the friendly and outgoing kid, might be attracted to baby Itachi's calm and somewhat mysterious presence. They could start by sharing simple things like toys or food. As time goes on, Naruto's determination and positive attitude could inspire Itachi, and Itachi's intelligence and calmness could influence Naruto in return.
One possible fanfiction could be about Sasuke finding a baby during his travels. The baby, having an unexplained connection to Itachi, starts to influence Sasuke's view of his brother. Sasuke might see the innocence in the baby and be reminded of the good in Itachi that he once overlooked.
Sasuke would likely become more protective. Usually Sasuke is very focused on revenge and his own power, but when faced with baby Itachi, he would feel the need to shield him from all harm. His cold exterior might soften a bit as he looks at the innocence of baby Itachi.
One idea could be that a villain's new invention accidentally zaps Kim and turns her into a baby. Her sidekick Ron has to take care of her while also trying to find a way to reverse the effect. He might struggle with basic baby - care tasks like changing diapers and feeding her, all while keeping her safe from the villain who might want to take advantage of her weakened state.
One possibility could be that an akuma's power goes wrong and instead of attacking Chat Noir in a normal way, it turns him into a baby. Ladybug is then left to protect this baby Chat Noir while also trying to find a way to reverse the transformation. She might have to take him to Master Fu for help, all while hiding him from Hawk Moth who could potentially use this situation to his advantage.
One great fanfiction with this concept is 'Baby Dean's Adventures'. It has a really cute take on how the other characters deal with a baby Dean. They struggle at first but then it becomes a heartwarming story of protection and family.
A common theme is protection. Itachi is often depicted as protecting baby Sasuke from various threats. Another theme is brotherly love. It shows their bond even when Sasuke is just a baby. Growth is also a theme, as we can see how Sasuke might grow up under Itachi's influence in the fanfiction.