As of now we know Galifianakis will be playing a hobo, Arkin is set to play a tour guide while Mr. Black will be kidnap victim.
The film, which stars Chris Cooper, Amy Adams, and Rashida Jones alongside Segal, is about a new Muppet named Walter who, along with Segal, aim to reunite the Muppets in an attempt to save the Muppet theater.
Now, if only scheduling wasn’t such a problem I’d say we would have this movie further along in the production process.
Source: The Hollywood Reporter