Yes, there are likely some fanfictions out there with such a concept. Fanfiction writers often come up with creative crossovers. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
I think Midoriya could be his best friend. Midoriya is kind - hearted and always looking to learn, which would appeal to Killua. They could bond over their shared desire to become stronger and protect others.
No, it's not appropriate. Fanfiction should focus on positive aspects such as character development, new adventures, or exploring relationships in a non - explicit way. Sexualizing a character in fanfiction goes against the spirit of most fandoms which are often family - friendly or at least should be respectful of all ages.
Often, baby Killua in fanfiction is presented as having a strong bond with certain characters from an early age. His loyalty and protectiveness, which are traits that are more prominent as he grows older, are sometimes hinted at in his childhood. He can also be quite endearing, with his small - child clumsiness and his determination to learn new things.
Maybe in the fanfiction, Naruto discovers Killua's world and is in awe of Killua's lightning - based abilities. He then tries to learn from Killua to enhance his own Rasengan or other jutsu. Also, it could involve the two of them teaming up against a common enemy that threatens both their worlds.