The ending of the Haikyu manga sees the main characters having accomplished their goals and leaving a lasting impact on the volleyball world. It wraps up their stories in a satisfying way.
One popular 'haikyu fanfiction' story could be those that focus on the character development of Hinata outside of the regular matches. For example, some fanfictions might explore his life at home, his relationships with his family members, and how that impacts his growth as a volleyball player. Another type could be the ones that create alternate universes where the teams have different dynamics or face different challenges in the volleyball world. There are also fanfictions that center around the friendships between the characters, like Kageyama and Hinata's evolving bond in new and unexpected scenarios.
One great Haikyu and One Punch Man fanfiction could be where the characters from Haikyu find themselves in the world of One Punch Man. They might be shocked by Saitama's incredible strength. Another could be a crossover where the volleyball skills of the Haikyu characters are used in a unique way in the One Punch Man universe, like to defeat some of the monsters in a really creative way.
The Haikyu anime follows the manga pretty closely. There are only a few tweaks here and there for better animation or to make it more suitable for the screen, but the essence remains the same.
The Haikyu anime adheres closely to the manga. The characters' personalities, the key events, and the overall tone are all preserved. Though there are a few changes here and there, they don't significantly deviate from the manga's essence.