Well, without more information about this 'sayonara novel', it's hard to say precisely. It could be a coming - of - age story where the main character has to say goodbye to their childhood innocence or a group of friends. It might also be a love story where two lovers part ways and the novel explores the pain and growth that follows this separation.
Sayonara fanfic could refer to fan - created fictional works that are related to the concept of 'sayonara' which means goodbye in Japanese. It might be fanfictions that deal with themes of parting, endings, or characters saying goodbye in a particular fictional universe.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
I'm not sure of specific'sayonara fanfic' examples off - hand. But it could be like a Harry Potter fanfic where a character decides to leave Hogwarts for good and there are a lot of emotional goodbyes with friends and professors. It would be a story centered around that parting moment.
I'm not entirely sure as I haven't read it in detail. But generally, it might be about piano sonatas and perhaps some kind of emotional story related to music, like a pianist's journey, the beauty and power of piano music, and relationships that are intertwined with the creation or performance of piano sonatas.
It could mean that a person who is a fan of 'Attack on Titan' (AOT) is reading or watching fan - created fictional works related to AOT. Maybe they are interested in seeing different interpretations or storylines that fans have come up with.
One way is to check on popular fanfiction websites. Look for ones with high ratings and a lot of reviews. For example, Archive of Our Own (AO3) has a large collection of AOT fanfictions. You can search for the most popular ones there.