I second the recommendation to cast Alexander Siddig. He was terrific in DS9, and recently in Cairo Time. He has such expressive eyes, he conveys so much with just a look. I would also love to see Tony Head (from Buffy the Vampire Slayer) in this production. I think he'd make an amazing Qyburn - he looks normal and kindly, but he can turn on the malevolent super creepiness at any moment and certainly could believably convey Maester-level intelligence.