A third idea is that the technology of the Terminator world somehow gets mixed with the magic and technology of RWBY. So there could be Terminators that are powered by Dust or have abilities that combine both mechanical and Dust - based powers. The heroes of RWBY would have to learn about this new threat and find a way to combat it, perhaps by teaming up with some of the human resistance from the Terminator side.
One idea could be that the T - 3000 teams up with a character from another universe to fight a common enemy that threatens both their worlds. For example, it could be a powerful alien force.
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 interesting concept could be a crossover with a time - traveling detective story. The terminator could be chasing a target through different time periods and the detective tries to stop it, adding a new layer of mystery to the story.
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.