I think this is by far the best candidate for a sequel. I see no point in doing sequels to Nausicaa or Laputa.

Less than the first half, to be more precise; it was only two-and-a-bit volumes' work of it that he had done before the film and the large majority of the elements the comic shares the film occur in just the first two (though even in those the story, character and world-building content are far, far richer; the only things about the film that really impressed me on home viewing were the title sequences and the appropriateness of the music the dreamy flashback scene is set to, though there is much animation-wise if you're into explosions and whatnot). I think it would be difficult for him to imagine a sequel to the Nausicaä film, not just a direct sequel but anything set within its continuity, having developed the comic's continuity for so many years since – something based on other parts of the comic, made irrespective of the existing film, would be possible, if rather unnecessary considering how often he's incorporated ideas trailed in Nausicaä into his own films. Indeed it's been said (I forget exactly where, I imagine I read about in a Nausicaa.net news abstract) that ANNO Hideaki, whom animated the most spectacular of those explosions, had wanted to make a film of the final story arc of the comic but wasn't given the rights – and he was was also the main director and writer of Nadia, which is perhaps the nearest we have to an expanded Laputa (though being sea- rather than air-travel oriented makes it as similar to Miyazaki's pre-Nausicaä Conan in other ways).

One of my first thoughts after experiencing or perhaps even while viewing Ponyo was that my wish if he were to do another feature is that it would be adult-oriented but at least as much of a cartoon feature. Which, come to mention it, Porco is the nearest he's done since his Lupin days, though I too found it unsuccessful: never here nor there enough between cartoon and manly comic-serial-adventure and the gross-out conclusion a real sour turn-off – a world away from the vitality of Ponyo I'd love for him to continue in. For these reasons and others I think I'd rather it were a new property, free from ties to an existing visual world, but the proof will be, as they say, in the pudding, and not what some of the recipe sounds like when read aloud. After ultimately disappointing with the previous two features the old guy shocked me in the best way with Ponyo, so it could go either way – a fresh re-design or trip down stodgy memory lane – judging from my experience.

Miyazaki actually has retired once already - he did so after finishing Mononoke hime. However, the untimely death of Yoshifumi Kondou (who was being groomed to be the studio's new, major director) shortly after brought him back.

A Porco Rosso-sequel sounds good to me - I like his quieter, more introspective movies better than his larger ones. What's not mentioned here, though, is that all this (including the fate of the studio) hinges on the box office performance of Karigurashi no Arrietty. So far, things are looking good, though: http://www.animenewsnetwork.com/news/2010-07-20/arrietty-draws-1-million-goers-1.4-billion-yen-in-3-days

If Miazaki is not interested in filming the rest of Nausicaa maybe he would consider letting his collaborators and admirers at Pixar do it?

Porco Rosso is my favorite Miyazaki movie. It has unique soul, deep soul within Miyazaki himself, who loves airplane very much.

Like few others mentioned before me I also like the idea about letting pixar do the sequel (would they? really?) but seing another Porco Rosso is like a dream come true.

I don't really care bout other movie tho I'd like to see another totoro.

Go Buta!