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.
The first date might be set in a forest. There are tall trees all around, and a gentle breeze blows through the leaves. They find a clearing where there are wildflowers. Naruto and Hinata sit there, surrounded by nature's beauty, and just enjoy each other's company.
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.
Naruto would likely become more stealthy. Instead of his usual brash and upfront fighting style, he'd have to learn to be quiet and calculating. His determination would still be there, but it would be focused on the precise and often secretive tasks of an assassin.
For Naruto, his character can be developed by showing how he adapts to the werewolf side of him. He might struggle with the more primal instincts at first but then learn to control them and use them to his advantage, just like he did with the Nine - Tails' power. Hinata can develop by becoming more assertive in this werewolf - filled world. She could use her Byakugan in new ways to detect threats or communicate with other werewolves.
Their relationship could develop as they are paired on dangerous Anbu missions. They would rely on each other for survival, which would deepen their bond. For example, Naruto might start to see Hinata in a new light when she saves his life during a mission.
Naruto could be a modern - day social activist, full of energy and a strong desire to make the world a better place, just like in the original where he wants to be Hokage. Hinata could be a shy but strong - willed woman working in a family - owned business. She might be a bit traditional in her values but also open to new ideas. Their different personalities would create interesting dynamics in a modern setting.
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.
For Naruto, you can show his growth in academics. He's always been more focused on physical skills, so in the academy setting, he could struggle with studies at first but then, with Hinata's help, start to improve. Hinata can be developed by showing her become more assertive. At the academy, she usually keeps to herself, but through her interactions with Naruto, she could start speaking up more. For example, when Naruto is wrongly accused of something in class, Hinata could find the courage to defend him in front of everyone.