No list here, just some comments on what's been posted so far
-- Since there hasn't been much high profile major projects for some time, this is a hard one because so MANY characters besides the major leaguers hasn't had a chance to do much. Characters like Scooter and the EM have pretty much come OUT of retirement but it still feels like they're retired because the quantity of their overall appearances are so scattered...take a character like Beauregard - he's been in on screen here and there but the quantity of his contributions have been so limited. With no regular major appearances or major projects that are any fewer than three years apart, even the characters that are "employed" still seem to be "retired" (mind you this is mainly in regards to the classic cast - Sesame is a whole other ballgame).
I don't mind Gonzo taking a lead role or for that matter being a figurehead for the Muppets as i actually feel that's a very natural place for him. Kermit is the leader and the one that's been around the longest, but while he represents the glue that holds all the craziness together, Gonzo represents best in one character the zaniness that defines the Muppets - the craziness that Kermit works so hard to manage. With Jim gone and Frank having placed other priorities in his careers over the Muppets, it seemed natural for Gonzo who always seemed to be fourth in top billing (after Kermit, Piggy, Fozzie) to come more to the fore ... but i actually think this still would have naturally and organically happened even with Jim was still with us and Frank was 100% devoted to the Muppets. Gonzo had actually been working his way to the number three spot past Fozzie slowly as early as the final seasons of TMS. I have no problem with Gonzo being on the frontlines...i just wish he was being built better these days (all the Muppets are very hard to duplicate and considering how complex his design is, it's pretty dang amazing that he's maintained a very consistent look - wardrobe changes aside - throughout the late 70's-early 90's. But somewhere around the beginning of the century, he's started to go off model - fur too smooth and ruglike and eye placement too crosseyed and eyelids a bit odd...and i don't know if it was just the way the puppeteer placed his hand in it, if they were using a "reject" or if the Disney people are really messing up the build of new characters, but i recently saw some of the footage from the French Muppets TV and wow he looked HORRIBLE (i haven't seen the latest stuff on the new muppets.com yet). When Dave Goelz and Gonzo were doing their presentation here in AZ it was the Gonzo that's been used most the last few years (with the eyes a little too crossed) but that looked SO much better than that sickly looking thing in those French clips.
And yes, Elmo has been overexposedx5, but regardless of the volume of his screentime, i don't even think it's been the constant overexposure that's ruined his character, but actually the quantity of his appearances - Elmo was a very charming enduring character once upon a time. But after Tickle Me Elmo, Kevin started focusing on some of the more aspects of the character - in a lot of cases, that may be considered character development, but in this case it has been character regression. The slightly shy sweet curious-in-a-reserved-manner Elmo is more brash and in-your-face (and combined with his redesign around season 30 where he started to look like he just came out of the automatic dryer didn't help either). If Elmo had the same overexposure today with the same personality/design he had in late 80's/early 90's i would probably not mind as much.
...Now after having said all that, i would really love to see more of Floyd, Janice, and Marvin Suggs. And though i wouldn't necessarily want to see the character revived without Richard who MADE her what she was, i really wish more would have been done with Gladys the canteen lady. Hey, i guess i've now named four so i may as well mention a fifth (though these are admittedly more "characters i wish i could see more" than ones i'd "like to see out of retirement" - but i would not only like to see Annie Sue used more but (and i know i'm going to raise some people's ire here) but if Louise Gold isn't available to play her (how long has it been since she's done some Muppeteering now?), there's enough strong female Muppeteers working now that i feel could actually play her well if Louise isn't available. (Or if that's too much for some people, at least get her back on screen in the background).