One common theme is the 'opposites attract' concept. Given sakusa's quiet and clean - freak nature and atsumu's more energetic and sometimes messy personality, this contrast often forms the basis of their relationship in fanfictions. Another theme is their mutual growth. They help each other become better players and people in many stories.
One popular type might be the ones that focus on their rivalry turning into friendship and then something more. For example, there could be a fanfiction where they start off competing for a spot on the team but gradually realize they have a deeper connection. Another could be set in an AU (alternate universe) where they meet in a completely different context like at a music concert and their relationship blossoms from there.
There are many out there. 'Opposites Attract' is a really nice one. It delves deep into how sakusa's reserved nature and atsumu's more outgoing and flamboyant personality play off each other. The fanfiction does a great job of creating situations where they have to rely on each other, leading to a beautiful relationship development.
Some Tumblr blogs also post or recommend Haikyuu fanfictions. You can search for Haikyuu fanfic blogs and then specifically look for Sakusa x Atsumu stories on those blogs. The bloggers often give a little summary or their thoughts on the fics, which can be helpful in deciding if it's something you'd like to read.
Probably Poseidon, as he is Percy's father and has a vested interest in his son's well - being. Zeus might also be protective in some cases, despite their past differences, as Percy is a hero who often saves the world which benefits Zeus's realm as well.
In some versions, Neville could also be one of Harry's protective mates. Neville, who grows in courage throughout the series, would be willing to stand up against any threat to Harry. And Luna, with her unique perspective and magical abilities, might also be part of Harry's protective group.
Their shared experiences play a big role. They've been through so much together, like countless battles and adventures. Percy has seen Annabeth in tough situations and he knows how strong she is, but also how vulnerable. So he feels the need to protect her. For instance, they might have had a close call in a previous adventure where Annabeth was nearly killed, and that memory makes Percy extra vigilant.
Perhaps Anko has a personal connection to Naruto. Maybe she sees a bit of herself in him, like his determination in the face of difficulties, and this makes her feel a sense of kinship and thus the need to protect him.
One reason could be that he sees Naruto as a younger version of himself in some ways. Both have had their share of loneliness and challenges growing up. Kakashi may want to prevent Naruto from going through the same pain he did. Another reason is Naruto's importance to the village. As the jinchuriki of the Nine - Tails, Naruto is a key asset. Kakashi, being a loyal ninja, wants to protect the village's future which is tied to Naruto.