I think the idea of the genre is good, and your problem could be solved easily, you can try to group the genres that are more alike... (i.e. heavy and power are not as different, as lets say black and power)...
Also, you can be a little unespecific (i.e. Instead of calling something sympho-melodic-power-astro-goth-glam-speed-prog metal, just put one), that's the problem with classifying metal IMO, the 10 different adjectives people put to each band when they classify it...
Also, most people agree that certian band is certain genre (i.e. I think most people agree that Therion is Symphonic metal, it's only a minority that that think it's Doom, or Trash, or whatever), and almost all the time a band labels themselves as some genre or another...
Also, another way you could do it is if you do it by country (i.e. Finland: Nightwish, Sonata Arctica, Stratovarius; Germany: At Vance, Edguy, Helloween; Norway: Dimmu Borgir, The Sins of thy Beloved, Theatre of Tragedy; Sweden: In Flames, Opeth, Therion), I think it could be easier...