Sitting here today, I really don't see any bands that in 10-20 years will be the Metallica/Priest/Aerosmith's/Zeppelins's of today : those bands that can last for decades.
It seems that our choices are mainstream "fad" music which doesn't seem to have longevity, or underground bands which have longevity, but only for those few who know of them.
I think bands like Korn, Tool, Nine Inch Nails, which have all made the mainstream will be around for some time (but are these considered "metal" by your definition?).
I think Metallica are the defining metal giants, whether you like them at present or not (personally, they faded to me after the Black album). But the future of metal can't rest on the shoulders of these "older" bands. There is a lot of good metal talent out there, but it seems that the music industry will not open their arms to them, so we are stuck seeing bands like Nevermore and Opeth in these small venues, while the other "crap" (as I call it) plays in the arenas.
It seems the only way to see these better metal bands at bigger places is when they play 1/2 hour sets with 20 other bands at these festivals. Crap - I'd love to hear a 2-hour Nevermore or Opeth set.