I view Labrie as a narrator conveying all characters on the regular album. Kind of similar to someone reading a book and altering his voice to reflect the lines of each character (audiobooks). And I think he does a fantastic job. However, I wouldn't say no to a full cast theater show, since the album definately fits that.
Here is my go:
Gabriel - Tommy Karevik
Ahrys - Damian Wilson
I want the two Ravenskill brothers to have more clean, melodious voices and both truly excel at that. Both have the power and range when needed.
Nafaryus - James Labrie
I love him as the Emperor on this disc, so let's keep him that
Daryus - Tom Englund
A mature and more darker gritty voice, with a sad undertone befitting this tragic yet villanous character.
Arabelle - Amanda Somerville
Faythe - Elyze Ryd
Arabelle is the guiding mother type of character, so I think Amanda's voice would fit greatly. For Faythe, I envision a more young and pop-sounding voice.
Xander - A talented kid from Broadway
Yes, if I select the voices close to what I think their characters may sound like, this is what I come up with. I think a kid sounds, well, like a kid (a tad older than Xander is supossed to be though, to have a more pleasant/warmer sound to the vocals).
Disclaimer: I have not focused on the lyrics of TA too much at this point, so I do not always know which character sings what line exactly (especially for disc 2).
Edit: totally missed the Karevik discussion above but yes, that's what I had in mind too