I think Ellen would be a pretty great cameo--like, maybe the Fraggles would walk by a TV store and see Ellen's talk show on all the screens.
Or maybe she'd walk by, notice them, and say to the person next to her, "Those are some funny-lookin' kids."
As for Abigail Breslin (the one from Little Miss Sunshine, right?), I think she'd be adorable alongside the Fraggles! ^.^
I don't know who'd be my last choice for a celebrity.
It'd probably do some good to add at least
one Muppeteer cameo.
~ Morgan "Confuuzzled" Goat