I just checked her out, slightly fearing that she was going to be another "rock belter" like Lzzy. I'm pleasantly surprised I must say, Anneke has a nice voice.
In general I feel, if they were to have extra singers (I don't think they will, but just for the sake of argument), it should be understated character singers. That is, singers who shine through their voice alone, not volume or metal raunchiness. The Lzzy Hale version wasn't bad, but in a lot of ways she is just a female version of James in the song. That other song with the female singer (forgot the name, the fan version) is so much better because it's the character of the voice that adds to the narrative.