Maybe the Warframes themselves start to take on some characteristics of the RWBY world. For example, a Rhino Warframe could gain some of the strength and appearance traits of a Beowolf. The story could then focus on how the Tenno adapt to these changes and how they use them to help the RWBY teams in their battles against Salem's forces. The fanfic could also explore the cultural differences between the two universes and how the characters overcome them to work together.
One idea could be that the Tenno from Warframe are sent to the RWBY universe due to a trans - dimensional rift. They team up with the RWBY characters to fight a new common enemy that is a fusion of the Grineer and Grimm. The Tenno can introduce their advanced technology and combat skills, while the RWBY characters show their unique Semblances and teamwork.
Another plot could be that a powerful relic from the Warframe universe is discovered in Remnant. This attracts the attention of both the Tenno and the various factions in RWBY. The Tenno must race against time to retrieve the relic before it falls into the wrong hands. Along the way, they form alliances with Ruby and her friends. The fanfic could include epic battles where the Tenno's Void powers are combined with the RWBY characters' Aura and Semblances.
One idea could be that the RWBY characters enter the Metaverse from Persona. They might have to face their Shadows, which could be related to their Grimm - fighting fears or personal insecurities. For example, Ruby could face a Shadow that represents her fear of not being able to protect everyone.
One idea could be that the RWBY characters find themselves in the Fallout wasteland. Ruby could use her Crescent Rose to fend off mutants. Weiss might struggle with the harsh and dirty environment compared to her usual clean Atlesian surroundings. Blake could use her stealth skills to navigate dangerous areas, and Yang's strength would be useful in combat against raiders. They could be on a quest to find a way back to their own world while also trying to make the wasteland a better place.
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.
One idea could be that the Servants from Fate end up in the RWBY world and have to team up with the RWBY characters to fight a new common enemy. For example, a powerful Grimm that has some sort of magic ability similar to a Servant's Noble Phantasm. Another plot could involve the RWBY characters being summoned into the Fate universe and having to participate in the Holy Grail War, but with their own unique skills and abilities that disrupt the normal flow of the war. A third idea might be that a mysterious force starts fusing the two worlds together, and the characters from both have to figure out how to stop it while also dealing with the cultural and power differences between them.
I think pairing Ruby Rose with a Volt Warframe would be really interesting. Ruby is all about speed and her scythe - wielding skills, and Volt is known for its speed - enhancing abilities. They could learn from each other's combat styles and become a super - fast duo on the battlefield.
One idea could be that the Jedi or Sith from Star Wars somehow end up in the RWBY universe. Maybe a Sith Lord is after the Dust in RWBY for its power - similar to how they seek the Force in Star Wars. The Jedi could then come to stop the Sith and end up allying with the RWBY characters.
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.
One possible plot could be that the Tenno from Warframe are somehow transported to the Star Wars universe. They could team up with the Jedi to fight against the Sith and the Grineer, who have also found their way into this new universe. The Tenno's unique abilities, like their Warframe powers and weaponry, would be a great addition to the Star Wars battles.
One idea could be the teams from Red vs Blue and RWBY getting mixed up in a shared mission. Maybe the Reds and Blues end up in Remnant and have to learn how to use Dust and fight Grimm while the RWBY teams have to deal with the Reds and Blues' unique military tactics. Another plot could involve a villain that is causing chaos in both universes and the combined forces have to stop it.
Maybe the RWBY characters are on a quest in Skyrim to find a way back home. Along the way, they meet various characters like the Dark Brotherhood or the Thieves Guild. Ruby might befriend a young Nord who is also an aspiring warrior and learn some of the local combat techniques. Weiss could use her knowledge of Atlesian technology to improve some of the weaponry in Skyrim. Blake could uncover some secrets about the Daedric Princes that are related to the Grimm in some way, and Yang could participate in the Skyrim's famous brawls and earn a reputation as a fierce fighter.