One idea could be that Sephiroth somehow gets transported to the RWBY world. He might initially be seen as a threat, but then the characters realize he has knowledge that could help against a new, powerful Grimm. For example, his combat skills and his understanding of magic - like materia in his world - could be adapted to the Dust - based magic in RWBY.
One idea could be that Sephiroth somehow gets transported to the feudal era of Inuyasha. He might initially be seen as a great threat due to his power. But as the story progresses, he could form an unlikely alliance with Inuyasha and his group against a common enemy, like a powerful demon that is even more menacing than any they've faced before.
Sephiroth is extremely powerful with his superhuman strength, speed, and his iconic Masamune sword. In comparison, RWBY characters have their own unique abilities. Ruby has her speed and Crescent Rose. But Sephiroth's overall power level might be higher initially. For instance, his ability to fly and use powerful magic attacks would be a big advantage.
One popular sephiroth crossover fanfiction might be one where he crosses over into the Final Fantasy VII Remake universe in a different timeline. This can create interesting storylines as he interacts with the new versions of characters.
Another theme could be the cultural differences between the two universes. The Final Fantasy world has its own set of values, and the Sekirei world has its own. Fanfiction might play with how characters from these different cultures interact and adapt. For example, Sephiroth might find the concept of Sekirei pairs very strange at first. It can lead to interesting character development and relationship building within the story. And this often makes for an engaging read as the author tries to blend these two very different worlds together in a believable way.
One great 'Final Fantasy Sephiroth Crossover Fanfiction' is 'Sephiroth in Another World'. It's really interesting as it takes Sephiroth and throws him into a modern - day Earth setting. The way the author describes his confusion and then his gradual adaptation is quite engaging.
Inuyasha would probably be very wary of Sephiroth at first. Inuyasha is used to dealing with demons and his own set of problems in his world. Sephiroth's otherworldly power and cold demeanor would make Inuyasha see him as a potential threat. He might even attack first out of instinct.
Well, perhaps in this crossover, Sephiroth is a rogue ninja from a far - off land that has powers similar to his in the Final Fantasy world. He starts causing destruction in the Hidden Leaf Village. Naruto, being the hero, decides to face him. Along the way, they discover that their powers have some strange similarities. For example, Sephiroth's sword techniques could be compared to some of the advanced sword - using ninja in the Naruto world. And they might end up having to team up against a greater evil that emerges because of Sephiroth's arrival.
Naruto would probably be very curious about Sephiroth. He'd likely try to be friendly at first, asking a lot of questions. Sephiroth, on the other hand, would be more aloof and might see Naruto as a nuisance or a potential threat. But as time goes on, Naruto's determination might start to intrigue Sephiroth.
Well, it could be about Sephiroth using his incredible combat skills from Final Fantasy in the world of Attack on Titan. Maybe he teams up with the Survey Corps to take on the Titans. He might use his Masamune sword to slice through the Titans' nape with ease, bringing a new level of badassery to the fight against the Titans.