Takeda is related to "The Small Giant" in some way.Their hairstyles look very similar,at least from behind; and given how all of the character designs are so unique, it's probably relevant.
Kageyama has Asperger syndromeTaken from the character's page, "He has difficulty expressing his emotions in a healthy manner, and understanding the emotions of others. Outside of volleyball he's a very literal thinker. Volleyball seems to be the only thing he's interested in." Difficulty in social situations and narrow interests are symptoms of Aspergers.
Oikawa is related to Nasujima-sensei.Both are tall (Nasujima an estimated 5'11"-6' and Oikawa 6'), handsome with similar features, and have antagonistic personalities. They also serve as the Butt Monkey of their respective series.