I'm gonna remain a bit skeptical with this, since I'm still waiting for
Lemony Snicket 2 (which hasn't been discussed since 2009), but I hope they are working on a non-sequel.
And I consider
The Muppets as a bit of a documentary, if you disregard everything after MTM, due to Kermit and Piggy's break-up at that point. (Though, now that I think of it, since there's no mention of when they break up in the finished movie, it's possible they broke up later.) While the other movies were outside the Muppet universe,
The Muppets is, if not anything else, a dramatization of the group's reunion. Ergo, Walter's inclusion in any future movies will not make them sequels, since he has joined the group as a Muppet. (And if anyone is still reading this, I applaud you, and shall soon reward you with a cookie.
)
And I wonder, since Disney was developing
The Cheapest Muppet Movie Ever Made at the same time that Stroller and Segel were developing
The Greatest Muppet Movie of All Time!!!, I'm curious if that's gonna be the next movie. And maybe to bring Walter into the mix, and pay tribute to the long tradition of fan-fiction, maybe it would be a script Walter wrote a few years ago, and Kermit decides to shoot with Gonzo directing, instead of having Gonzo writing and directing it all as it was originally planned.
(I know I know a bit too much about TCMMEM, but I spent my whole weekend on Muppet Wiki.
)