This is an awful idea. If i were you, I'd think first...LED ZEPPELIN and Metallica...First of all you need to compare this 2 bands as musical I guess. Well, I agree Metallica WAS a huge, successful band, but they are so insufficient near a legend like Led Zeppelin. That would be so ridiculous. I'd say, "damn, that could be awsome, what an idea" if this topic was something like; Led Zeppelin and Deep Purple, Led Zeppelin and Ronnie James Dio or maybe Led Zeppelin and Jeff Beck-Eric Clapton...I hope I could tell what i wanted to say..