Sherlock Holmes is a great choice. His deductive skills would contrast well with Kaito Kid's thieving skills. Another good one is Lupin III. They are both master thieves, and a crossover between them could be full of interesting heist scenarios. Conan Edogawa would also be interesting. Since he's always trying to catch Kaito Kid in the original series, a crossover could explore new dynamics between them.
In a Kaito Kid crossover fanfiction, characters like Batman would be amazing. Batman's detective skills and gadgets would make for an exciting confrontation with Kaito Kid. Also, Sailor Mercury from Sailor Moon. Her intelligence and water - based powers could create unique interactions. And L from Death Note. His brilliant mind could be used to try and outwit Kaito Kid.
The main character is of course Kaito Kid. And then there's Conan Edogawa who often tries to catch him. Nakamori Ginzo is also an important character as he is always chasing after Kaito Kid.
Well, if you're doing a crossover with 'Pokémon', Ash Ketchum would be a great addition. He could help the Rugrats understand the world of Pokémon and they could go on a Pokémon - hunting adventure together. Another good character for a crossover, say with 'Digimon', would be Tai Kamiya. He could show the Rugrats how to interact with Digimon. And for a crossover with 'Ben 10', Ben Tennyson could be included. He could use his alien forms to protect the Rugrats in dangerous situations.
Hermione is always a great character to include. As a kid, she could be shown solving even more complex problems than in the books. Her intelligence and determination can drive the plot forward in fanfiction.
Sally Jackson, Percy's mother, is also an important character. Her love for Percy and her strength can be a central part of the story. She could be the one who gives Percy the emotional support he needs when he's confused or scared about the strange things happening around him. In a fanfiction, she could also have some secrets of her own related to the mythological world that Percy discovers as he grows up.
Definitely Sasuke. His relationship with Naruto is so important in the original story, and when it comes to a fanfiction with kid Naruto and Boruto, Sasuke can act as a mentor or a challenging rival for both of them. Another great character is Hinata. She has a special connection to Naruto, and in a fanfiction, she could play a role in guiding Boruto or providing support to kid Naruto in a new situation.
A cool Kaito Kid crossover fanfiction idea is with the world of Harry Potter. Kaito Kid could try to steal a magical artifact from Hogwarts, like the Philosopher's Stone. He'd have to navigate through the magical defenses and deal with the wizards and witches. Also, a crossover with One Piece. He might target a Devil Fruit, which would put him in conflict with the Straw Hat Pirates. And a crossover with Pokémon. He could be after a rare Pokémon that has some special power, like Mewtwo.
His butler, Kuroba Toichi, if still alive in the fanfiction world, would surely come to his aid. He has a lot of experience and resources, and his loyalty to Kaito Kid would make him a natural choice to help him when he's hurt, whether it's by providing medical assistance or getting him out of a dangerous situation.
Shikamaru is a smart choice. His intelligence can be used to solve problems that Kakashi, Naruto and the others face. And his lazy - but - actually - really - smart attitude can provide some comic relief. Also, including the Third Hokage can give the story more depth, as he is the leader of the village and has a lot of knowledge and influence over the kids' lives.
The parents should be included. Their parenting styles and reactions to the girls' different situations can add depth. Mrs. Rooney's attempts to keep the family together while dealing with Liv's Hollywood drama and Maddie's sports - related issues can be really interesting. Mr. Rooney's comical yet supportive role can also be explored further. Additionally, Willow from Liv's world could be a good character to bring in. She could befriend Maddie and introduce her to the more glamorous side of Liv's world, causing Maddie to have a new perspective on her sister.
Shadow would be a great addition. His brooding nature and powerful combat abilities would add an interesting dynamic. He could be suspicious of Infinite at first and be in conflict with Sonic about whether to trust him or not. And then there's Knuckles, who could guard some important location or object that is related to the crossover event, like a powerful chaos emerald that is causing the dimensions to cross.