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.
Another interesting concept is that Sephiroth is a long - lost relative of one of the RWBY characters. This could bring in a whole new dynamic of family secrets and hidden powers. He could be related to a character like Ruby, and his presence could unlock some new abilities in her that she didn't know she had.
One interesting idea could be a crossover of different timelines within RWBY. For example, characters from the past meet the current ones and have to deal with the differences in their worlds. Another could be a crossover of different teams in unexpected ways, like Team RWBY having to team up with a rival team to face a common enemy that is more powerful than any they've faced before. And sometimes, there are crossovers that involve alternate versions of the characters, where their personalities or abilities are slightly different, which can lead to very unique and engaging storylines.
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.
One interesting idea could be the unique character interactions. For example, how the RWBY characters might interact with the denizens of 'heaven' in this fanfiction. Maybe they learn new skills or gain different perspectives on their own world.
One idea could be that Superman arrives in the RWBY world and has to learn about Dust and its powers. He could team up with Team RWBY to fight a common enemy, like a powerful Grimm that is immune to normal attacks but not to Superman's strength. Another idea is that Ruby discovers she has some Kryptonian-like abilities and Superman helps her understand and control them.
One idea could be to cross RWBY characters with characters from another popular anime, like Naruto. Imagine Ruby with the ability to use Naruto's shadow clone jutsu. Another could be a crossover with the Harry Potter universe, where the RWBY characters attend Hogwarts and have to deal with magic and dark wizards. Or perhaps a crossover with the DC universe, with RWBY characters teaming up with the Justice League to save Remnant from a greater threat.
For 'rwby multi crossover fanfiction', a crossover with 'Fullmetal Alchemist' would be interesting. RWBY characters could interact with the alchemists and learn about alchemy. They might even find ways to combine their Semblances with alchemy. Also, a crossover with 'My Hero Academia' would be exciting. The RWBY heroes could train with the students at U.A. High and exchange combat techniques and strategies.
Another interesting aspect could be a story where the Grimm from RWBY are somehow affected by the technology of the MMZ world. Maybe the Maverick virus gets mixed up with the Grimm essence, creating new and more dangerous hybrids. The characters would then have to figure out how to stop these hybrids while also dealing with the cultural differences between their two worlds.
One idea could be to combine the unique weaponry of RWBY with the grimdark setting of Warhammer. For example, Ruby's scythe could be enchanted with some sort of Warhammer - style magic or runes. Another idea is to have the characters from RWBY face off against Warhammer - style daemons or chaos warriors. This would really test their skills and teamwork. Also, the concept of semblances could be related to the powers in the Warhammer universe in some way, like a character's semblance being a form of psychic power in the context of Warhammer.
One idea could be to have the characters from RWBY enter the Fate universe and participate in a Holy Grail War. Ruby could be a Saber - class servant with her scythe as her weapon. Another idea is to have the Fate characters visit Remnant and help the RWBY team fight the Grimm. For example, Archer could use his projection magic to create weapons for the RWBY team.
One idea could be a crossover with another popular anime like Naruto. The RWBY characters could find themselves in the Naruto world and form relationships with the Naruto characters, creating a harem dynamic. For example, Ruby could befriend Naruto and his friends, and some of them might develop romantic feelings for her over time. Another idea is a crossover with a game world, such as the Elder Scrolls. The RWBY girls could explore the vast lands of Tamriel, meet different characters, and have a harem of followers or admirers from different races in the game. Maybe Weiss could attract the attention of some noble elves or powerful mages in Skyrim.