Well, a 'naruto x rwby fanfic' could center around a training arc. Imagine Naruto training the RWBY team. He could help Ruby improve her speed and combat skills by teaching her some of his ninja movement techniques. Yang could learn from Naruto how to better control her temper during battles, similar to how Naruto had to control his Nine - Tails' chakra. Meanwhile, the Naruto characters could also be learning about the Grimm and how to fight them. They could incorporate their ninja tools and jutsu to combat these new and strange enemies from the RWBY world.
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.
Maybe the Forerunners in the Halo universe somehow cross paths with the Dust - the energy source in RWBY. This could lead to a power struggle or an alliance based on the unique properties of Dust. The characters from RWBY might use their Semblances in combination with Halo technology for some really cool battles.
One idea could be that Naruto characters are transported to the RWBY world. Naruto himself could befriend Ruby and they could learn from each other's combat styles. For example, Naruto's chakra control might inspire Ruby to better control her own energy. Another plot could involve the Akatsuki from Naruto being interested in the Dust in RWBY's world and causing chaos to obtain it.
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.
A possible plot could be that there is a strange energy disturbance that starts to merge the three universes together. Characters like Ragna from Blazblue, the Persona users, and Team RWBY find themselves in a strange new world that is a mix of all three. They must figure out what is causing this and how to stop it. Maybe there are also new enemies that are created from the combination of the different mythologies and technologies of the three universes. For instance, Grimm with the ability to use Persona - like powers or Blazblue - style Ars Magus. It would be a great opportunity for character development as they learn to work together despite their differences.
One idea could be a crossover where Naruto's world and the world of X - 23 from the X - Men collide. Maybe X - 23 is on a mission and accidentally ends up in the Hidden Leaf Village. She has to learn to adapt to this new place with its different culture and ninja ways. Naruto could be intrigued by her and her abilities, and they could team up to face a common enemy.
Perhaps there could be a situation where the Grimm from RWBY start to invade the world of Genlock. The Genlock team, with their diverse skills and abilities, would have to learn about the Grimm from the RWBY characters in order to fight them effectively. And in return, the RWBY characters could learn about Genlock's technology and combat strategies. For example, they could figure out how to use the Genlock's mech - like suits in their battles against the Grimm. It would be a great opportunity for character development as they interact and adapt to each other's ways.
Well, a cool concept in RWBY x Undertale fanfic could be the exploration of the different worlds. RWBY has its own complex world with Grimm, Huntsmen, and Kingdoms. Undertale has the Underground with its distinct areas and monsters. A fanfic could involve characters traveling between these two worlds and the culture shock they experience. Also, the magic systems in both could be merged in interesting ways. The Dust in RWBY and the magic in Undertale might interact in ways that create new powers or abilities for the characters.