You can find 'Fire Force fanfic' on fanfiction websites like Archive of Our Own (AO3). There are many users who post their creative works about 'Fire Force' there. Another place could be FanFiction.net, which also has a collection of fan - made stories related to the series.
Well, there's a 'Fire Force fanfic' called 'Flame's Redemption'. In this fic, a character who has made some mistakes in the past tries to make up for it within the context of the Fire Force world. It's quite interesting as it shows character growth and development. Also, 'Fire Force: Hidden Powers' is a fanfic that explores the idea of some characters having undiscovered powers.