Friendship is a big theme. Despite their differences, they had a strong bond at one point, and fanfics may develop this aspect more, showing how their friendship could have evolved in different scenarios.
Yes, there are some. You can start by looking on fanfiction websites. Some might be based on different interpretations of their relationship in the original work.
Well, 'A New Path' is also a great 'Griffith x Guts' fanfic. It takes a different approach by imagining a world where certain events in the original story didn't occur. This changes the dynamic between Griffith and Guts completely, and it's really interesting to see how their relationship develops in this alternate universe.
The appeal lies in the fact that the original story leaves a lot of room for interpretation. Fanfic writers can take these characters and put them in new situations, whether it's exploring their relationship in a more romantic light or in a different world entirely. It gives fans a chance to see their favorite characters in new and exciting ways. For example, some fanfics might focus on how they could overcome their differences and form a stronger bond. Also, the unique personalities of Griffith and Guts, with Griffith's ambition and Guts' more rugged nature, create a lot of potential for interesting storylines in fanfics.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.