Besides what has already been confirmed, I think that they should all have Muppet Morsels, deleted scenes, and original theatrical trailers. I also think that The Muppet Movie shoudl have the screen test that was on the first DVD (for those who don't have the first one).
Additionally, The Muppet Movie should have a commentary track with jerry Juhl, jack Burns (if he is still alive), James Frawlry, Paul Williams, and kenny Aschter (if he is still alive), another commentary track with the performers (Dave Goelz, Jerry Nelson, Steve Whitmire, and hopefully Frank Oz, kathy Mullen, and maybe even Caroll Spinney), and maybe a commentary track with Kermit, Fozzie, Gonzo, Miss Piggy, and Scooter.
The great Muppet caper should also have three commentary tracks: one with jerry Juhl, Craig Sherman, and Brian henson, one with Frank Oz, Dave Goelz, jerry nelson, Steve Whitmire, and Kathy Mullen, and one with Kermit, Fozzie, Gonzo, and Miss Piggy.
The Muppet Christmas carol should have a secodn commentary track with Dave Goelz, Steve Whitmire, and hopefully Frank Oz, jerry nelson, david Rudman, Louise Gold, karen prell, and jerry Juhl, and if a muppet commentary coudl be done, then one with Kermit, Gonzo, Rizzo, Miss Piggy, and cameos by Bean Bunny, Fozzie Bear, Bunsen, and Beaker.
Muppet treasure island should have an extra commentary track featuring Steve Whitmire, Dave Goelz, Bill Barretta, and hopefully Kevin Clash, jerry Nelson, and Frank Oz. Maybe John Henson could do commentary with them.
I wonder how funny it would be if Statler and Waldorf did audio commentary on any of the DVDs.