If Frank Oz left the Muppets because he didn't want his career to entirely be defined by them, and he wanted to have more time with his family and directing, then why would he jump back in, regardless of role? I can't see it happening.
There seems to be some very personal aspect to his work with the Muppets. When they started the Ernie and Bert sketches back up, there was some sort of hitch in Frank's Bert to Steve's Ernie. Not a coldness, not an air of dejection... just the microscopic awkwardness of doing something with someone else after years of doing it with a friend. Not quite as awkward as the Curly Joe Stooge Shorts, but there was something off about the chemistry. Now, compare that to when Frank performed Grover when Jerry was Fat Blue... he still got it.
But for the Muppet part, there was something very off about his characters in later films, MTI and MFS especially. Even if you take the dubbed dialogue into the equation. Fozzie became an idiot in MTI, nothing like Fozzie at all. And Piggy only had a couple genuine Piggy moments in MFS.
As far as this movie went...
we got the best possible Muppet Movie under the circumstances, and considering how other family film revivals are done, I'd say we got off pretty lumping lucky. Frank's too safe comment does ring true to one thing. This was the first movie that was a
continuation of another Muppet movie, albeit a loose one (unless you count the fan nugget of Doc Hoppers in VMX). Now, there's a reason why GMC and MTM were completely different movies from TMM, and a reason why JHH was completely different from TMS, which was mutually ended in season 5. Jim liked doing different things. That's why Muppets were basically TV specials in the mid-80's. Jim wanted to build more of a brand for the Creature Shop at the time. TM
is backwards in the sense that the writers and director wanted a nostalgic Muppet film that brought together the three films AND the TV show, combining them in a way Jim wouldn't have done, only because that's already been done. HOWEVER, to build the brand back up after the last 3 theatrical films, they
needed to work that nostalgic angle. A High concept like "Cheapest Movie" would scare away anyone but diehards. And then we'd just have that one film, like Superman Returns, that was
supposed to build a brand back up but failed to do so.
And above all... Disney could have given us a CGI, realistic looking crapfest that made the Yogi Bear movie tolerable. I wouldn't want to seem ungrateful with the film they made, because the possibilities of what could have been are far more distressing.