Hmmm... Tentatively, based on just vocals...
Gabriel - Michael Eriksen (Circus Maximus) (I think his voice is pretty close to Gabriels and he fits the 'feel' of the role)
Arhys - Tommy Karevik (Seventh Wonder, Kamelot, Ayreon) (He would be fantastic as any role, really. He's very versatile and theatrical)
Xander - Uhhh... Young Michael Jackson? (haha can't think of any kid singers... either that or just cast some whiney emo type singer)
Faythe - Tarja Turunen (Ex. Nightwish) (Difficult to cast Faythe because we never hear a female voice in her role, but Tarja would be good)
Nafaryus - James Labrie (I think he was awesome in this role, but he was great in all his parts on this album)
Arabelle - Anette Olzon (Nightwish) (Arabelle doesn't have a lot of lines, so this one is flexible)
Daryus - Russell Allen (Symphony X) (Some people already suggested him as Nafaryus but I think he would be better as Daryus)
Yeah, theres my list. Subject to change. It would be cool to see how often certain singers show up in each role. I'm pretty confident in all of these, except Xander and Nafaryus (because I would rather cast someone entirely different here, not that JLB isn't good).