If Donna's coming back, I wonder if they'll get Moira Kelly in to play her, rather than Lara Flynn Boyle. Lara was with Kyle McLachlan for a bit (allegedly insisting that Cooper and Audrey never hooked up as she was too jealous of Sherilyn Fenn), and she's apparently not the easiest person in the world to work with (plus, as noted above, she looks a right mess these days).
When Dana Ashbrook confirmed, he said he'd been talking to Sherilyn Fenn on the phone, and she's in too, so I guess that's Audrey unofficially confirmed as well.