I've long thought that the best way to do the foam parts on Gonzo would be injection-molded hard plastic, just like we saw on the Palisades action figures. The problem with that, though, is that making those molds is incredibly expensive (from memory we're talking around $10,000 here) and there would have to be a whole plastic skull created too to support the structure. Add in the additional engineering costs of getting his eyelids to move (because the outcry on here if they didn't move would be painful) and it just becomes cost prohibitive.
I do think, though, that the best approach for most of the Muppets would be to have an internal vac-formed plastic skull. Sure they wouldn't feel the same as the real thing, but it would allow for precision in actually getting the head shape 100% right. It also means that there will be little to no change in form over time (Kermit's body, for example, feels like a very dense foam, and I was under the impression that all foam will eventually start to degrade - can anyone correct me on that?) I wouldn't mind in, say, 20 years, having to restuff Fozzie's body since the shape would be fairly easy to replicate to an acceptable point. His head, however, would be much harder to get just right, and that is where (for me at least) 90% of the likeness comes from.
Then again, I'm no engineer and I'm probably thinking of things that are cost prohibitive (although I did think vac-forming was fairly economical?) or just wouldn't work construction wise. Any puppet-makers out there care to weigh in?