Some fanfictions could focus on the relationships between characters from the two series. Picture Ruby Rose from RWBY becoming friends with Shinra Kusakabe from Fire Force. Their different outlooks on life and their shared determination to protect people could make for a heartwarming friendship story, with them learning from each other and growing as individuals.
One interesting RWBY and Fire Force fanfiction could be a crossover where the characters from both universes team up to fight a common enemy. For example, the Huntsmen and Huntresses from RWBY could join forces with the Fire Force soldiers to combat a powerful Grimm - like infernal hybrid. This would create unique battle scenarios and character interactions.
You could write a fanfiction that delves deeper into the relationship between Arthur and Shinra. Maybe they get into a big argument over their different approaches to being a hero. Shinra believes in using his powers for the greater good in a more practical way, while Arthur is all about living out his 'knight in shining armor' fantasy. This argument could lead to them going their separate ways for a while, and then having to come back together to face a powerful Infernal that neither of them could defeat alone.
One interesting 'rwby greek fire fanfiction' could be about a character in RWBY who discovers an ancient Greek fire weapon and has to learn how to use it without getting burned by its power. Maybe they face off against a new type of Grimm that is only vulnerable to Greek fire.
Well, you could create an OC fanfiction where the OC is a scientist who discovers a new way to combat Infernals that doesn't involve burning them. This OC might be initially rejected by the Fire Force but then proves their method's worth. Also, an OC with a special connection to the Adolla Burst could be interesting. Maybe they can communicate with the spirits of the Infernals or have visions related to the Adolla realm, which would lead to new adventures and revelations within the Fire Force world.
One idea could be a character from RWBY gaining the power of the Speed Force and using it to outmaneuver their enemies in combat. For example, Ruby could use the speed to create afterimages and confuse Grimm while she attacks them from unexpected angles.
Another interesting plot could be a tournament where teams from both RWBY and Battle Force 5 compete. But as the tournament progresses, they start to realize that there is a hidden conspiracy behind it, and they must work together to expose the truth and stop those behind it.
One interesting concept could be the combination of different character classes from Fire Emblem with the unique abilities of RWBY characters. For example, a RWBY character might inherit the skills of a Fire Emblem knight, like high defense and proficiency with heavy weapons. Another concept could be the exploration of the different kingdoms in RWBY through the lens of Fire Emblem's strategic warfare. Also, the idea of semblances in RWBY interacting with Fire Emblem's magic or special abilities is really fascinating.
Fanfiction.net is also a well - known platform. While it may not be as organized as AO3 in some ways, it still has a lot of RWBY and Fire Force fanfiction. You might have to do a bit more sifting through the stories, but it can be a great source. Look for stories with high ratings and positive reviews to find the better ones.
One idea could be that Naruto's ability to control chakra is somehow combined with the Fire Force's pyrokinetic abilities. For example, Naruto might teach the Fire Force members some of his chakra control techniques, which could enhance their ability to control their flames in a more precise way.
One idea could be that Ghost Rider somehow gets transported to the world of Fire Force. He could team up with the Fire Force members to fight against a new type of infernal that has powers related to the underworld. His Penance Stare might be used to expose the true nature of some of the more mysterious infernals or even some corrupt individuals within the Fire Force organization.
One idea could be that the characters from RWBY are transported to the world of Fire Emblem. They could join forces with the Fire Emblem characters to fight against a common enemy like a powerful dark dragon. The RWBY characters' unique abilities would add a new dynamic to the battles in the Fire Emblem world.