SNAP! I get to play Metal Pimp!
These are all bands that I love, who I think would fit right in with your vision of Powerfest. For any who don't know them:
Ram-Zet - dark, thrashy prog metal, with male death vocals, female death vocals, female clean vocals, and female operatic vocals. I've lost track of them over the last year or so, but I think they were going to release a new disc soon (if it's not out already). I always go apeshit when I listen to them.
Dominion - I know Rob knows this band... they're long-defunct, and they were very underground, so they'd probably be the most impossible band to get from this list. They used elements of power, prog, and doom metal, and had male death vocals and female mid-range clean vocals (I can't stand the super-high operatic stuff that you get from bands like After Forever and the like).
Dismal Euphony - used to be a gothy doom band with female operatic vocals... now they're more like an In Flames meets Lacuna Coil type of band, with powerful mid-range female vox.
That takes care of the female stuff...
Bal-Sagoth - super-pompous, over-the-top, Rhapsody-esque power metal, with strong black metal elements added in. Features cheeseball narration (narrator's voice is different from disc to disc) and raspy black metal vocals. Highly melodic, yet creepy at the same time.
Mezarkabul - used to be known as the Turkish band Pentagram (and are still known by that name in Turkey). Thrashy/doomy power metal that ALL fans of Morgana Lefay and Tad Morose need to check out; in fact, Maleficium is my second favorite cd ever, and Pentagram - Anatolia is my third favorite.
Xentrix - UK thrash band from the 80's/early 90's which is, apparently, reforming for some reunion shows. Old Metallica-esque, with some Pantera influence on their later material.
Scanner - originally a Helloweenie clone, they started getting darker, more intricate, and more experimental as they went on, and now they're one of the most original-sounding German power metal bands in existence (when they actually release music, that is). Mental Reservation is in my top ten albums ever.
Veni Domine - doom metal band with very Geoff Tate-ish vocals. Their last album was uber-lame, but I believe they can still bring it live, if they play older material.
Stratovarius - they're this little band that nobody's ever heard of.
Seriously, most people who bitch about them being too repetitive, or too sugary-melodic, or too unoriginal, haven't heard the first three albums. There were some gems on there; too bad Timo hates that stuff now.