I'm not exactly sure what this is asking. I know what a cross over is, but I'm not sure if it's asking to cross one of the Ghibli film with another Ghibli or with something else.
so, I will go my own path. I would love to see a cross over with Sapphire and Steel and Howl's Moving Castle. The thought of Steel coming up against Calcifer would be quite the hoot.
Strangely enough, someone commented on this story, which I have totally forgotten that I'd written. Link takes you to AO3.