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.
Iruka could be included. He was Naruto's first teacher and has his own connection to Kakashi. His presence can bring out the different teaching styles between him and Kakashi. Another good character is Jiraiya. His relationship with Naruto as his godfather and his connection to Kakashi through being comrades can add some interesting storylines.
Sasuke could be a great addition. He could be a rival to Naruto even at a young age, and Kakashi would have to deal with their constant bickering. His brooding nature would add an interesting dynamic to the story.
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.
Iruka - sensei. He was Naruto's first teacher and has a connection to Kakashi as well. He could provide some emotional support or some important backstory information that helps Naruto and Kakashi in their journey in the fanfic.
There's also 'Kid Kakashi's New Friend Naruto'. This fanfiction delves into how Naruto, being the outcast he is in the village at first, finds an unexpected friend in Kid Kakashi. The author does an excellent job of depicting the insecurities of both characters and how they overcome them through their friendship.
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.
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 Naruto himself, since it's a drunk Naruto fanfiction. His friends like Sasuke and Sakura should be included too. Sasuke could react in his typical stoic but secretly concerned way when Naruto is drunk. Sakura could be the one trying to get Naruto to sober up or at least stop making a fool of himself. Kakashi would also be great, as he could have some humorous reactions to Naruto's drunken antics.
Naruto is often very well - written in these fanfictions. His determination and growth are usually highlighted. For example, in some stories, his loyalty to Kakashi is shown in a very touching way, like when he refuses to leave Kakashi's side even in the face of great danger.