I would love for there to be another official fan convention...but there would be two things that world really mar any future fests:
(1) The first fest was before Muppets were sold to Disney so it was (rightly) a Muppets & Jim Henson Company Fan Event...incorporating not just the Classic Muppets but also Jim's early films/projects, Creature Shop, Fraggle Rock, Bear (though at least that would still be Disney), Farscape. Even though there may be panels which would get into talking about other Henson stuff as sidenotes to other anecdotes, it's highly unlikely you'd see a good mix of Muppets PLUS other non-Disney owned Muppet stuff. The only way i could see this happening if it was a joint effort between Disney and the Jim Henson Legacy. But realistically i think you could see Muppet conventions and even Henson Company (minus Muppet) conventions but nothing like the original where you had all of Henson history represented, celebrated, and appreciated.
(2) If Disney agreed to an official Muppet fan convention (and they probably would call it something else besides Muppetfest largely due to above paragraph), it would be organized/run by Disney - not Creation who did the original fest plus all the Farscape cons...they do the official Star Trek cons and basically that's what they do. They're the pros. Even though they're far from perfect and people have had various issues of different sorts through the years, they know what they're doing and have years of experience. Disney would probably keep things very corporate and especially OVERPRICED in all areas. As much as i am slowly being won over by Disney's handling of the Muppets (under Lyelle Breier's leadership), i still don't have high opinions of Disney in its other aspects and certainly don't picture them putting on a convention that was fun, accessable, reasonably priced, and really "magic". I see them doing things like the cruises which only really rich and elite people can do, but not something that would be reasonable for the average fan (and by "average" i mean in terms of economy/"regular people" - ie the people here on these boards...we're all "extraordinary fans" and that's who a convention should be geared towards - those that live and breathe Muppets, not those that kinda sorta enjoy them but go on a Muppet Cruise because they can splurge on that...but could never tell you the difference between a Fraggle and a Frackle and couldn't spell Fozzie's name right (if they even know the bear's name and don't call him Rowlf or Beeper)
So that's the bad news. The good news is i think Disney would be open to doing a fan convention especially if it was timed around the climax of its Muppet development plans - they can show footage of all these huge Muppet fans at this convention every time a news organization reports on the opening of the new movie.