Well, it's a bit of a mystery. There's no definitive evidence that Kurt Cobain was a die-hard comics fan, but there are hints that he had at least some appreciation for them.
No. There is no evidence or indication that Kurt Cobain was ever supposed to be in 'Pulp Fiction'. The two are from different creative worlds. 'Pulp Fiction' has its own distinct cast that was carefully selected by Quentin Tarantino, and Cobain was mainly associated with the grunge music scene, not with this movie's production or casting plans.
Kurt Cobain was not slated to be in Pulp Fiction. The casting and character lineup for the movie didn't include him. It's just a misconception or rumor that may have circulated for no valid reason.
Good Kurt Cobain fanfic often has a deep understanding of his music. For example, it might reference the lyrics of Nirvana songs in a meaningful way. It also usually tries to capture his complex personality, his shyness mixed with his powerful stage presence.
One interesting Kurt Cobain fanfiction could be about an alternate universe where he overcame his struggles and continued to make music into his old age. It might explore how his music would have evolved and the new collaborations he could have had.
Obviously, Kurt Cobain would be the main character. His bandmates from Nirvana, like Krist Novoselic and Dave Grohl, might also be important characters. There could also be characters representing his family members, friends, and people he interacted with in the music industry.
No. There's no indication that Kurt Cobain was considered for any Quentin Tarantino movies. Tarantino typically works with actors who are part of the acting community, and Cobain was a musician. Their worlds didn't really intersect in terms of casting for Tarantino's films.