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.
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.
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.
Crossovers between 'Sherlock' and 'Elementary' can be interesting too. Imagine Sherlock Holmes from the BBC's 'Sherlock' and Joan Watson from 'Elementary' working together on a case that spans both their versions of New York City. It would be a clash of different interpretations of the classic characters.
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.
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.
In this crossover fanfiction, perhaps Sephiroth is brought into Rias' world. Rias' peerage might initially be very wary of Sephiroth. However, his skills in combat and his knowledge of magic could prove useful. Rias could see potential in him and try to sway him from his darker path. They might go on adventures together, facing off against powerful devils and other threats, and in the process, a unique bond forms between them that is a mix of friendship and mutual admiration.
A potential storyline could be a battle - royale - style plot. Sephiroth and Rias are forced to participate in a tournament in a neutral dimension. They have to fight against various powerful opponents. Along the way, they learn about each other's fighting styles and motives. This could lead to an alliance between them as they realize they have a better chance of winning together. And in the end, they might face a final boss that is a threat to both their existences.
A good 'Final Fantasy Sephiroth Crossover Fanfiction' should have a believable crossover concept. For example, if he crosses into a different universe, the rules of that universe should be well - defined and Sephiroth's presence should make sense within it.
One possible plot could be that Sephiroth, with his superhuman abilities and thirst for power, somehow ends up in the Marvel universe. The Avengers initially mistake him for a new, extremely dangerous threat. They try to fight him but quickly realize his power level is off the charts. Then, they might try to find a way to send him back to his own world while Sephiroth, intrigued by the power sources in the Marvel universe like the Infinity Stones, tries to get his hands on them.