With Studio Ghibli's Arrietty the Borrower hitting the UK, the Guardian profiled director Hiromasa Yonebayashi. Addressing what's next, the suggestion of previously mentioned/dismissed second Porco Rosso movie came up again. "Keeping it in the Ghibli family: [Yonebayashi is] working on Miyazaki's sequel to his 1992 feature Porco Rosso."
On a possibly related note, Ghibli Wiki just reported that Ghibli's still very much involved former president Toshio Suzuki responded to a question about whether the studio's next film will be one from Hayao Miyazaki, saying " I don't know what it will be, but probably so."
Its accuracy has been criticized, but still invaluable, Nausicaa.net posted translated highlights from a 2010 Cut magazine interview with Hayao Miyazaki in which his discussed a "Last Sortie" for his beloved pilot pig Porco Rosso.
"I'm not thinking about a movie of a girl."
"I think that I must think about only a movie of a boy."
"I do not need to make a movie if it is not a tragic story of a boy."
"So I want to escape to "Porco Rosso: The Last Sortie". I have all its materials."
"It should be interesting."
"It is set for Spanish Civil War."
"If next two films succeed and [studio president Toshio] Suzuki-san lets me make it (Porco) while saying, 'It cannot be helped because it's a hobby of the old man', I'm happy. It's my hobby."
Meanwhile, Studio Ghibli co-founder Isao Takahata is also at work on a project;Tale of the Bamboo Cutter last anyone heard.
Japanamerica author Roland Kelts recently commented on Twitter "Was just @ #Ghibli yesterday: Takahata's working on a new one, too. Couldn't get anyone to confirm [it's Tale of the Bamboo Cutter]. Just that Takahata's in the studio & working, albeit ... slowly, as is his wont."