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.
Speculation on NationalsThe endgame arc could proceed like this: Karasuno gets to Nationals as the Miyagi rep. Since the Tokyo prefecture is permitted two representatives, both Fukurodani and Nekoma also make it to Nationals for Tokyo. Presumably, Karasuno plays Fukurodani, among others, but the final showdown to win it all will be between Karasuno and Nekoma - the fabled "Battle of the Trash Heap" will happen at last. Considering Karasuno and Nekoma are Foils both in individual characters and as teams - Karasuno is stronger offensively, Nekoma's defense is a force to be reckoned with - being able to defeat them will fully demonstrate Karasuno's growth as a team.
The "Small Giant" is the lovechild of Kageyama and Hinata sent to the pastPosting some theories floating on Tumblr. In a literal sense, yes I am talking about Timey-Wimey Ball Mr Sea Horse bullshit but I want you to consider this theory more metaphorically, and rather that "Small Giant" is representative of what Kageyama and Hinata can achieve when they work together.
Hinata's Bathroom AdventuresHinata will run into Sakusa and Kiryuu (the #1 and #2 aces in Japan) during a trip to the bathroom at Nationals.