I'm with Dr. Tooth on this one. The idea of cramming as many characters as possible into the last film worked because of the nostalgia factor and the relaunch concept, particularly insofar as they were re-mounting The Muppet Show. (The sight of Link Hogthrob just chatting away with Bobby Benson as everyone else buggers up their cues for the opening theme song still cracks me up.)
This time, however, we don't have the luxury of the nostalgia factor to yank everyone in. The Muppets will sink or swim on their own merits, so it won't work to have 30 or 40 (let alone 300 or 400) main characters with their own individual plotlines. Bundle it up in a nice little package and let's go from there.
Don't forget, as well, that the Muppet brain trust (pre-and-post-1990) has always found a way to work dozens of characters into the proceedings, even in films that seemed to zero in on a fixed number of familiar faces (TMM, GMC, TMTM, MCC, MFS, VMX, TM11), so I'm sure that'll be the case here as well.