Tough roles to fill will be Avon and Servalan.
I could see Alex Kingston as Servalan, but who could be Avon.
The only person I've seen lately who I've got an Avon vibe from is actually Peter Dinklage.
Kingston might actually be too good a fit as Servalan, if that makes sense, as would Baccarin.
Left field suggestion from me would be Amy Acker. I'd ignore the camp/glam side (Because even Kingston can't quite do it like Jacqui) and focus on the manipulative side, and Acker's a good enough actress to convince as a doe eyed innocent who flips into a cold hearted killer at a moment's notice, then smiles sweetly as she stabs you in the back.
I hope they keep Blake as a fantatic...