I hope that Jerry Nelson, Dave Goelz, Steve Whitmire, Karen Prell, and Kathy Mullen can all perform in this. I also hope that Cheryl wagner can perform Ma Gorg, and that as many of the background performers as possible will be able to perform. I think David Rudman could perform Junior Gorg. I don't know about Gunge.
However, I think that most of the main fraggles, assuming that this movie isn't a prequel or featuring competely new characters, will have different performers.
Jerry Nelson has limited his work to just Sesame Street. I don't know if he would want to go through the work of performing his fraggle characters again. Especially sinc eGobo, in my opinion, was the star of Fraggle Rock. I guess he coudl just do voices if he can't/ won't actually perform.
karen prell has a contract with Pixar, and she is allowed to do occassional performing if she wants to. I am not sure if a movie would get in the way of her Pixar work or not.
I don't think Kathy Mullen has done any performing for henson in a long time, but hopefully she will work in this (then again, she didn't speak in the season 1 DVD interviews).
I am sure that Dave Goelz and Steve Whitmire would do this if they can schedule their time around the Muppets. I know that Disney is trying hard to get the muppets, especially Kermit, out to the public, and there are many Muppet productions currently being worked on.
Note: I have not read very many of the previous posts, so i do not know if anything I said has been discussed or covered.