Japanese magazine Cut talked to Hayao Miyazaki recently, and in the resulting interview (helpfully translated by Nausicaa.net and reported by AICN) he revealed that he's planning a sequel to 1992's Porco Rosso, titled Porco Rosso: The Last Sortie.
The film was the story of a 1920s / 1930s fighter pilot who unfortunately had the face of a pig. He's a former World War I flyer, but in that story worked as a sort of bounty hunter, guarding ships in the Adriatic from pirates and spending his evenings hitting on a hotel owner, Gina.
Miyazaki told Cut that this sequel would see him flying in the Spanish Civil War (1936-1939), which would seem a perfect fit. After all, Miyazaki's depiction of aerial warfare and blitzkrieg in films like Howl's Moving Castle reflects the account of survivors of that war rather well. Just one slight problem: most Italian pilots in Italy at the time were there as part of Mussolini's contribution to the Nationalist (ie more-or-less* fascist) cause, which would mean they're kind of the bad guys. Hopefully Porco Rosso will be a volunteer in the Republicans' International Brigades instead.
Miyazaki's going to work on developing the film over the next two or three years, but that doesn't mean we'll see it that quickly. It will be four or five before we're likely to see anything onscreen.
*OK, history grads, by all means argue the toss on whether Franco was a true fascist below, but for the sake of simplicity, that's how we're labelling him here.