One common theme is friendship development. Since Hinata and Kenma are from different teams, their relationship evolving from acquaintances to friends (or more) is often explored. Another theme could be how they support each other's growth in volleyball.
Sure. You can try searching on Archive of Our Own (AO3). There are a lot of great 'haikyuu hinata x kenma fanfic' there. Just type in the relevant tags and you'll find many interesting stories written by fans.
One great 'kenma x reader fanfic' is 'Kenma's Quiet Love'. It beautifully captures Kenma's introverted nature and how he gradually opens up to the reader. The author does an amazing job of creating a sweet and believable relationship.
One common theme is isolation. In these fanfictions, Kenma often isolates himself from his friends and teammates due to his depression. Another theme is the struggle to communicate. He finds it hard to express what he's going through. For example, in many fanfics, he'll keep his feelings bottled up until something forces him to open up.
One great 'kenma x reader fanfiction' to start with could be 'The Quiet Moments with Kenma'. It really captures Kenma's introverted nature well and how the reader can gradually break through his shyness. It has a really sweet and slow - burn kind of relationship development that many fans enjoy.
Well, Kenma is such an interesting character. He's quiet and reserved, which gives fanfic writers a lot to explore. His love for gaming also provides a unique backdrop for all kinds of stories.
They might be really surprised. Some characters could be amused by the creative scenarios fans come up with. For example, Hinata might be excited to see himself in all kinds of wild adventures in the fanfictions.
I have no definite answer. Sometimes manga series end when the story feels complete to the creator or when publishing circumstances change. So, we'll just have to wait and see.