I'm not sure I can offer something that fits every single requirement of yours, but here goes...
Flaming Row - Elinoire
I won't even bother to describe it, loads of different singers, compelling plot. If you liked Ayreon, just go get it. This is awesome.
Kamelot - Epica / The Black Halo
It kinda revolves around one character, but there are four characters in the whole concept overall, and it's excellent power / progressive metal anyway. Very strong plot relying on the spiritual and philosophical themes, like discovering meaning of life and all that. It really fits all the requirements except the characters: the protagonist, Ariel, has the most vocal lines in the story and the other characters have only some short parts here and there. Um, maybe not so fantasy, but then again, The Astonishing and The Human Equation aren't fantasy too.
Avantasia - The Scarecrow / The Wicked Symphony / Angel of Babylon (called "The Wicked Trilogy")
Really excellent, features lots of different characters voiced by different singers and genre styles, but has quite an abstract plot. I mean, it's traceable through all the songs, but you have to pay attention and put your imagination to use to make sense of the story. Regarding "fantasy", see Kamelot's write-up.
Avantasia - The Metal Opera
Classic power metal through and through, this has somewhat cheesy fantasy plot yet a handful of characters and great musicianship.
Avantasia - The Mystery of Time / Ghostlights
Another story with mostly unfollowable plot (if you don't have the booklet with the story for the first album, you're screwed, and the second album doesn't even feature the story in the booklet), but really great music and different characters voiced by different amazing singers.
Beyond the Bridge - The Old Man and the Spirit
Well, this isn't fantasy and it features only two characters (and another character in passing), but this is excellent anyway. Some romance involved and really strong plot overall, one of my favourite concept effors.