Acoustic metal.
Well, it's my first metal effort of any sort (let alone acoustic metal), so it's fairly juvenile. But you can get a very good idea of the direction I'm talking about. Every sound is acoustic except for a bit of feedback from the mic on one or two songs. Check out Not Through Learning pt2, it's probably the best example. This album is kind of metal/post-rock/noise, oh and there are 3 folk songs on it ('cause that style's so easy for me). My next one will be pure metal.
I think there's a lot of potential for acoustic metal. The acoustic guitar is fucking ace and modern music only taps a tiny little bit of its potential. It can play hard music that is typically given only to the electric guitar. Acoustic and electric guitars aren't all that different. My example of some very fierce, fluid and 'electric' acoustic guitar music is Bob Dylan's self-titled debut album. I mean, it's a folk-blues album from 1962. But it's harsh. Not heavy, but very intense.
When I started getting into metal one of the first things I looked for was acoustic metal but Impaled Northern Moonforest was all I could find. I don't consider a band like Agalloch or Opeth to have much to do with acoustic metal -- they're just bands that include non-metal acoustic parts.