violin: I'm game, but only if it still sounds 70's proggy like Opeth and not like Tristania/Sirenia/3rd and the Mortal etc (I like those bands but I don't want Opeth to sound like them).
cello: would be a great element to play with. there's a USBM project called Azrael that uses cello below the guitars for a grinding, unsettling effect. really cool.
sax: actually there is a black metal band using sax, they're called Oblomov and they're from the Czech Republic. it's only used in small doses, as solos, and it's pretty weird. surprisingly, it works. check out "Lost in Emotions" or "Starsend" off the album Mighty Cosmic Dances.
I wouldn't mind hearing some more types of acoustic instruments for the softer sections, maybe taking cues from the dark country/Civil War sound of 16 Horsepower? I know Peter Lindgren likes that band.
-gg
np: Floyd - Animals