Well, a cool plot for a Fate RWBY crossover fanfic could be that a Grail appears in the RWBY world. This Grail starts to corrupt the Grimm, making them even more powerful and intelligent. The RWBY teams then have to seek help from the Fate side. Maybe they summon a Servant like Saber, who teaches them about the concept of the Grail and how to combat its corrupting influence. Or perhaps there are some RWBY characters who have a hidden connection to the Fate world, like Ruby having some latent magical abilities that are awakened when the two worlds start to interact. This could lead to a whole new adventure as she tries to master these new powers while still fighting the Grimm.
In a Fate RWBY crossover fanfic, a possible plot could be that the Fate characters are transported to Remnant. The RWBY characters are initially suspicious of them. However, they soon realize that the Servants' powers could be useful in their fight against Salem. For instance, Archer's projection ability could be used to create weapons to fight the Grimm. On the other hand, the RWBY characters' unique Semblances also intrigue the Fate characters. Blake's ability to create clones could be seen as a form of illusion magic in the Fate world. This exchange of powers and knowledge could form the basis of a great story as they work together to face the threats in Remnant.
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.