that's a pretty tall order there mate...
I have not yet come across a band that could be comparable to Opeth...
If you are looking for stimulating music I can recommend a few that have a hard time leaving my cd player. Kyuss-groovy desert rock, puts you in a real other state of mind. Government Mule-blues driven, definitely not metal, but good nonetheless.
When I was in Australia, I caught a couple of shows by the band 'Alchemist' and was quite impressed.
Gorefest's second to last album 'soul survivor' had a bit of a prog feel to it, but never made it onto my favourites list. You could try listening to Otis, or Sons of Otis, as I think they are now called. Not much speed to it, but a cool sound.
Another band, that is probably something more like my old man would listen to, is Steely Dan. Nowhere near metal, but if you put it on during sex, you can't help but feel just a little bit like Dirk Diggler.
Can't say I really know your musical preferences, so it's hard to make a recommendation. If you wanted to check out Chthonic, try these sites. Sorry it's not much, but they're the only decent sites in English. Their Chinese name is 'shan ling' (means shining spirits) and in my own humble opinion they are at the top of the Asian Metal scene. I wouldn't give the guitarist any medals, but the traditional chinese instruments, and female vocalist make the band something special.
http://www.excelpro.com.hk/catalystaction/PRODUCT/slblack05.htm
http://www.excelpro.com.hk/indie_c/month/chthonic.html
The first album 'where the ancestors souls gathered' is an epic about the first mainlanders coming to Taiwan and conquering the aboriginals. Some of the songs are told from different perspectives (ie. from that of the aborigal's gods) but if you can't read chinese you will have a hard time understanding it. The new album, which I haven't yet heard, apparently has one english song on it. enjoy.