On one hand, kind of a shame that The Wind Rises won't be his final work, cus it really works great as a final film. But on the other hand nobody is going to complain about getting a new miyazaki film. However it is worth noting that from all that I've read about it so far "Boro the Caterpillar" looks to be closest to the cutesy family kind of Miyazaki films. So we'll more likely be getting another Ponyo rather than another Princess Mononoke. Now don't get me wrong, personally I love Ponyo, but I know most people don't consider that to be among his best and probably don't feel like we need another one. We'll see I guess.
There's also the possibility that he could die during the production of every film he makes now. Unfinished work sucks, so I hope it doesn't happen, but he'll probably be pushing 80 by the time this new film is done so it becomes more and more likely.