One possible crossover character could be Harry Potter. Imagine Kid Kakashi in the wizarding world, learning about magic and using his ninja skills alongside it. It would be a really unique combination. Also, Superman could be a great addition. The comparison between Superman's superhuman abilities and Kakashi's ninja techniques would be fascinating. Kakashi might have to use his intelligence and stealth to keep up with Superman's raw power.
Naruto himself could be a great crossover character. Since Kakashi is his mentor in the original series, seeing them interact when Kakashi is a kid could be really interesting. They could have a different kind of dynamic, with Naruto perhaps being the more experienced one in some aspects. Another potential character is Goku from Dragon Ball. The contrast between Kakashi's ninja skills and Goku's martial arts and superpowers would create a really exciting crossover.