Niecy Nash as Star? I know she often gets confused with Sherri Shepherd, but I think Nash can capture both Star's intelligence and her over-the-topness. Pam Grier might work here, too, as a less conventional choice. .
Miley Cyrus as Debbie Matenopoulous?
If they want to go (and pay) super A-list, they can try to get Helen Mirren or Glenn Close to play Barbara. Jennifer Grey would be a less conventional choice, but I think she might be able to so it.
Susan Sarandon or Mary Steenbergen might also generate a lot of interest as Meredith. Geena Davis or Jane Seymour could work, too.
Whoopi would be extremely hard to cast. Like her or not, she has carved out a singular presence as an entertainer. The person who played her would have to be extremely talented and okay with pissing Whoopi off. Cynthia Ervio could do it, but she looks up to Whoopi. Maybe Sheryl Lee Ralph? Or Alfre Woodard? (Okay, @Primetimer. Alfre Woodard it is).
I think Vivica Fox could be a good Sherri. Phoebe Robinson could also work.
Joy should be played by Fran Drescher. Michaela Watkins is my distant second choice.
Rosie is another important hard-to-cast role. Meryl Streep? (Just kidding) Megan Mullaly might work. Bette Midler would be awesome, but I don't think she'd do it.
Ming-Na Wen is too old to play Lisa, but then again, she seems to own a time machine, so she might work. Constance Wu or Chloe Bennet are better matches.
Jay Mohr as The Viewmaster/Bill Gedde.
Iliza Shlesinger or Nikki Glaser as Elisabeth Hasslebeck. Nikky looks a lot like Elisabeth to me. Jennifer Lawrence might be able to work it, too.