Hm. I was actually thinking about this not long ago. I actually wish that Miyazaki had expanded some on his films. It seemed that after every film, he always left a bit of a cliffhanger. Like, it left you with a bunch of questions. Spirited Away could have had a good sequel to see if Haku really did return to Chihiro and what happened if/when he did, you know? I think that's the one I wish could have had more to it then what he gave us.
Ponyo also would have made a good sequel too. It would have been interesting to see how much Ponyo grows with Sosuke. Also, I think it would give us a lot more on how she sees the world. I always related to her because she struck me as havign a bit of autism (mind you, I don't normally like diagnosing characters). That's why it would have been really neat to see her view on the outside world she becomes a part of. As well as seeing how she grows with Sosuke and if they ever actually fall in love for real or not. I mean, they were both about four or five in the movie, so it wasn't real love... just puppy love.
Sadly though, there won't be anything from him anymore, considering he retired. :( I can't blame him for it or anything. I mean, he is getting to that age where he feels it's for the best.