One idea could be that Naruto, who somehow ends up in the RWBY universe, teams up with Blake. They could share their experiences of being outcasts in their own worlds. Naruto's determination and his never - give - up attitude could inspire Blake, and in turn, Blake could show Naruto the unique Grimm - fighting skills she has.
Another possibility is a story where there is a threat that requires both Naruto's chakra - based powers and Blake's semblance. For example, a powerful new type of Grimm that can only be defeated by combining Naruto's Rasengan with Blake's shadow clones. This would force them to train together and learn about each other's abilities and cultures.
A fanfiction could be written about a school project in Volume 1 that Ruby and Blake are paired up for. At first, they struggle to work together because of their different personalities. But as they research and present their project, they find common ground and start to see each other in a new light, leading to a closer relationship.
Maybe the Kyuubi and a Grimm in RWBY have a strange connection. The Kyuubi's power might be the key to either destroying all the Grimm or, in a more complex plot, it could be that the Grimm are actually drawn to the Kyuubi's power in this new world, causing chaos and forcing the RWBY characters to make difficult decisions about how to handle it.
Another plot could involve a RWBY character ending up in the Naruto universe. Say, Weiss gets sent to Konoha. She might initially struggle with the different culture and the use of chakra, but eventually she could use her Glyphs in unique ways by combining them with chakra. This could also lead to some interesting interactions between her and the ninja students there.
One interesting idea could be the combination of Naruto's unique ninja abilities with the RWBY characters' combat styles. For example, Naruto's shadow clone jutsu might be used in creative ways in the RWBY universe, perhaps to confuse enemies during a battle. Another idea could be the exploration of how the different power systems, like chakra in Naruto and aura in RWBY, interact. Maybe a character with Naruto - like powers has to learn to adapt to using aura as well.
One common theme is the clash of cultures. Naruto comes from a shinobi world and Blake from the RWBY universe, so there's a lot of exploration of how their different values and ways of life interact. Another theme is friendship. They often start as strangers and gradually become friends as they face challenges together.
One idea could be Ruby and Blake getting lost in a mysterious forest during a mission in Volume 5. They have to rely on each other's skills, Ruby's speed and Blake's stealth, to find their way out. Another could be that they discover a long - lost relic together and have to protect it from a group of Grimm.
Perhaps Salem discovers a way to use Naruto's chakra for her own evil purposes. So, Naruto has to find a way to stop her while also dealing with the new and unfamiliar world of RWBY. He might learn some new skills from the RWBY characters along the way.
Perhaps in this fanfiction, Weiss discovers a strange portal and steps through it, ending up in the Naruto world. She meets Naruto and is intrigued by his ninja abilities. They could start training together, with Weiss learning some ninja techniques and Naruto learning about Dust. Their combined skills could then be used to protect Konoha from a new threat that has some similarities to the Grimm from her world.
One idea could be that Naruto and his friends get transported to the RWBY world. They team up with Ruby and the gang to fight a common enemy that combines the threats from both universes, like a group of powerful ninja - Grimm hybrids.
One idea could be a fanfiction where Blake's ears get injured during a battle. This would not only affect her physically but also emotionally as her ears are such an important part of her Faunus identity. Her friends would then have to help her deal with the injury and the resulting self - doubt.