I think since I barely know about devil-man besides watching some episodes from devil-man crybaby on netflix and some stuff I read on the wiki, I would be unfit to write a crossover between Devil-Man and RWBY but I have enough knowledge for a basic plot involving Yang turning into a devilman immediately after having her arm cutoff in Volume 3 because when The Dragon Grimm emerges it awakens the demons laying dormant in Mountain Glenn. Ruby becomes scared of Yang due to her transformation which hurts Yang but she still tries to protect her little sister, Yang then discovers Salem's plan and decides to form a Devil-man Army by finding others like her. Iron-Wood being paranoid wants all demons shot on sight.
Yang's demon form will be appear like a mix between a harpy and a wyvern having traits of a human, bird and dragon as well as wings that double as arms, her human form will be how she looked before due to her strong will being the reason she becomes a devil-man. Remnant will actually be a future earth with yet another recreated humanity and faunus will be descended from demons that gained a more stable humanoid form.
So that's basically my idea in a nutshell, so any writer who sees this could make a story out of it.