I think that a TV series would be the best for the franchise as a whole, it would give a great chance for character development and giving all the Muppets a spotlight as opposed to having to focus on a few in the movies. I don't know that Disney has enough faith in them to give them one right now, even though ABC is desperate for any sort of successful program right now. I don't see them fitting in on either ABC Family (all their original programming are dramas) or Disney Channel (too young an audience), so it would be ABC or bust.
As for the format of the show, I don't think a variety show would be a good idea because it would make them too dependent on each week's guest star. For every John Cleese or Glenda Jackson the original show had there was a Gladys Knight or Liberace. So I think a sitcom would be the best idea. Perhaps each season could be built around a theme (the Muppets are developing a Broadway show, the Muppets have been given a talk show, etc.) so that you could sprinkle in musical numbers and guests, but keep the focus on the Muppets.
Will any of this happen? Who knows. I'd like to see Disney dip their foot in the water by giving the Muppets a Muppet-centric special, perhaps a real Christmas special (compared to the Lady Gaga focused one from last year). If the special does well, then try another one and go from there.