Every time a major project comes up, fans have their checklists of what characters they're rooting to see in there. We hope they'll be a whole slew of well known and obscure but when all is said and done, the fact is someone's going to be disappointed simply because you can't do everything in a movie. More than you can in a special, sure (look how much they squeezed into LTS) but not a whole much more (only about another 30-45 minutes). Any "one shot" production will only be able to do so much - it's regular continuing projects like television series where characters can really flourish and grow. Ultimately, with a movie, the main goal is to best utilize the characters best suited to tell that tale.
We really know little about this project casting or even story wise other than the one-sentence concept revealed years ago: "Gonzo, put it charge of directing a Muppet movie, blows the budget on the opening titles and is forced to do the rest as cheaply as possible." Casting go go just about anywhere, since one would figure the movie would be (much like The Muppet Show) just as much about the backstage drama as well as the final movie the Muppets make (with different Muppets probably more in behind the camera duties and others in front).
Remember that at present this seems like it may have little if anything to do with Segel's "Greatest" movie (we don't even really know if Segel's going to be involved in the writing of this one) - so anything anyone heard about Segel's concept including the reviews of the script from aintitcoolnews or toughpigs are now off the table, including characters scheduled to appear and how much they're in it.
Whether or not these characters are (a) in the movie and (b) have significant parts in it doesn't mean that they won't be featured in other projects...sure the Muppets being discussed here recently all had casting issues in the 90's, but things have changed. The people behind the Muppets eventually realized letting the characters live on is a stronger tribute to their creators than silencing them (VMX was a huge sign to the fans that "we're bringing your favorites back" and that also seems to be a big part of Disney's vision as well). Scooter is fine - he's had healthy screentime in many of the major Muppet projects in the 00's despite having been played by at least three different people during that time; the band in its five-piece entirety (sorry, Lips fans) is back - and while Rowlf has been underused in some projects like MWOO or VMX, he's still on-camera, he's not only been the star of a viral video, but also had major parts in Statler & Waldorf: From the Balcony and Red & Green Christmas album. He may have had a small part in LTS but he was very much part of the overall ensemble both playing the piano and singing at the end.
In between all these screen projects, we also have the comics on a regular basis trotting out all our favorites and even giving us cameos from long absent favorites. Now that may not seem like much since they're different writers and in a printed media - but let's not forget they're keeping the Muppets out there on a regular basis between the screen projects and (re)introducing all these characters to people all over again - the team that brings us the felt and fur Muppets i'm sure read these and notice - any Muppet writer worth their salt will keep abreast of how the Muppets are being represented in all official areas regardless of if it's stuff from their own pen or not. (And since many reviewers - in and outside the Muppet fan community are saying the comics are the most Muppety the Muppets have been in ages, i'm sure it's safe to say the "regular" Muppet writers are taking notes!)
Bottom line - it's fun to speculate on what a major new project will bring, but let's not go overboard on worrying about if everything/one we may want to see will be in there.