Spoilers for the anime are unmarked.
Takeda is related to "The Little 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.- Jossed. The Little Giant finally debuts in the present timeline for Karasuno's Nationals quarterfinal match. His name is Tenma Udai and is unrelated to Takeda.
Kageyama has Asperger syndrome
Taken 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 Nationals
The 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.- Karasuno, Fukurodani and Nekoma do indeed succeed in their respective qualifiers (with the latter teams making up 2/3 of the Tokyo representatives)note and go to Nationals. As for the progress of their matches, they are a little different from the above scenario. The arc is also not the final arc, as the series explicitly makes a Time Skip and spends the remainder of the story focusing on Hinata's Brazil training and V-League match with Kageyama.
- Karasuno beats Shiratorizawa in a whopping five sets to become Miyagi's representative for Nationals. At the tournament, they do not get to play Fukurodani (who is in the opposite bracket) but do play Nekoma, albeit in Round 3 instead of the final. They bow out in the quarterfinals again Kamomedai, the team of the new "Little Giant" Hoshiumi.
- Nekoma qualifies as the third Tokyo representative after Itachiyama and Fukurodani (losing to Fukurodani but beating Nohebi to get that third slot). The "Battle of the Trash Heap" with Karasuno does happen but in in Round 3, with Karasuno winning in three sets. Not only that, but Karasuno beating them doesn't so much signify their growth as it hammers in how evenly matched they are, since eventually the game is won by a setter error due to sweat on the ball rather than individual skill or tactics.
- Fukurodani qualify as the second representative for Tokyo (beating Nekoma but losing to Itachiyama), and the Nationals arc shows them making the semifinals, with a win over the No. 4 seed Mujinizaka (Kiryuu's team) along the way. They sadly do not get to play Karasuno (whom they could only had faced in the final since they were on opposite brackets), due to that team's quarterfinal loss. While this arc ends without showing Fukurodani's remaining matches, the final arc confirms via a flashback that they do make the final but lose to a previously unmentioned team in five sets.
The "Little Giant" is the lovechild of Kageyama and Hinata sent to the past
Posting some theories floating on Tumblr. In a literal sense, yes I am talking about Timey-Wimey Ball and Mister Sea Horse bullshit but I want you to consider this theory more metaphorically, and rather that "Little Giant" is representative of what Kageyama and Hinata can achieve when they work together.- Unconfirmed, most likely jossed (the metaphorical theory, to be clear). The series never properly explains the Little Giant's skills so direct comparisons can't be made, but his existence is only of importance to Hinata (mainly that he should develop his own identity as a volleyball player and not just follow his idol), not Kageyama.
Hinata's Bathroom Adventures
Hinata will run into Sakusa and Kiryuu (two of the top three aces in Japan) during a trip to the bathroom at Nationals.- Jossed. Kiryuu and Sakusa's teams Mujinizaka and Itachiyama unfortunately lose to Fukurodani and Inubushi respectively before Hinata can meet them. Sakusa and Hinata's first meeting is confirmed to have happened after the two join the same pro team (several years after Nationals) and is a fairly mundane one.
- Amusingly, after the Time Skip, Sakusa does join Hinata (and Bokuto and Atsumu) during his bathroom encounter with Kageyama, Ushijima and Hoshiumi right before their V-League match.
Kageyama and Sawako Kuronuma are related.
Dark hair, a scary face, and general cluelessness might somehow run in the family. That said... Nami Hayase is Kageyama's mother.
Her relationship with You Hazuki mirrors that of Kageyama and a certain other overenthusiastic red haired volleyball rookie.