Yes, yes, and yes! Those are all ideas that, not only would I like to see, but they seem like the most likely candidates in the event that another theatrical movie would be made.
Unfortunately, as much as I would like to think that the general public would flock to see a Pigs in Space or Electric Mayhem movie in theaters, harsh reality hits me and I realize that those movies would most likely fail unless they had Piggy and Animal plastered all over them. I think that those films would work better as direct-to-video releases than theatrical. If we're going to have Muppet movies with just Muppets, I have this bad feeling that not too many would be interested.
Unless, of course, they got cameos that would be actually fitting to the plot. Get Buzz Aldrin (or even some Star Wars characters, again) in Pigs In Space, Bill Nye or Neil Degrasse Tyson in the Muppet Labs movie, and some classic-not-tweeny-bop band in the EM movie and they'd be set.