If I may jump into this conversation, I think that Walter's prominence should depend on what kind of production is being produced.
If there was to be another TV Show (and I know this is not likely), then I think that Walter could become a major character without diminishing the roles of others. With a new episode week after week, there are plenty of opportunities to showcase multiple Muppets. Each episode could have a handful of Muppets that are featured more heavily, and they all kind of take turns. This is what happened in The Muppet Show. There were some episodes without major characters like Piggy, Fozzie, Gonzo, etc. Even Kermit was reduced to a cameo appearance in the Nancy Walker episode, but the show as a whole doesn't feel like anyone was stepping on anyone else's toes.
If it were a movie or a TV Special, which is more likely, then I'd say Walter probably shouldn't be front and center. When there's only an hour and a half to two hours for the story, and we have so many Muppets that fans want to see, it can be difficult having Walter in a main role. Even when I was writing my fanfic, I started out with Walter joining the main action throughout the story, but I found that I didn't really have much for him to do, so I changed it up and gave him a small moment in one of the chapters. This seemed to work well. I don't want to see Walter disappearing forever, but I don't see him rising through the ranks that quickly.