Well, I was surprised about the Nevermore track(s) that feature female vocal...something in the same reign would be nice on an Opeth cd. They're so diverse in their musical styles/genre so I don't think a female backing vocal (or just for a short period of a song) would hurt or make them look any different than how most people look upon them now.
Not many bands in this genre feature bossanova rythms, jazz, classical, slap bass etc. etc. and blend it all in as beutifully as Opeth does...(my guess is, they'd make the female vocal part fit in JUST as nicely...) if it where ever to happen...