Off the top of my head:
Stars who've already proven their "Muppet cred": Weezer, OK Go, Brad Paisley, Stephen Colbert, Jason Segel, Jack Black, Amy Adams, Jim Parsons, Kristen Schaal, Sarah Silverman, Zach Galifianakis, Jimmy Kimmel, Tina Fey, Emma Stone, Jim Carrey, Robert De Niro, Tom Bergeron (and the Dancing With The Stars judges in a cameo role, including Len Goodman sitting with Statler and Waldorf in the box), David Arquette, Kathy Griffin, Kylie Minogue.
Stars that could work well with the Muppets even though (to my knowledge) they haven't officially done so yet: "Weird Al" Yankovic, Jon Stewart, Tom Hanks, Tim Allen, Eric Stonestreet (I'm told he had a cameo that was cut from the recent movie - what a shame), Matthew Morrison, Amy Poehler, Jane Lynch, Jennifer Hudson, Kirsten Dunst, Julia Roberts, Reese Witherspoon, Kirsten Wiig, Kaley Cuoco, Melissa McCarthy.
I would want Jeff Dunham to be as far away from this type of project as possible. He has a fan base, but I think his sensibilities are better suited to Henson Alternative than The Muppets.