I don't think that its possible to top the cover of Total Eclipse of the Heart featuring the guys who smashed appliances for percussion, that was pretty epic.
why the fuck would they cover opeth's weakness. An opeth song would be sick because they're legendary and death is way over covered but not weakness...
Hocus Pocus by Focus would be awesome, that is a great heavy, jamming tune.
Johnny Cash, The Who, Yes, Elton John, wait... what can you guys afford to cover? That's the real question! You've gotta pay for rights, and royalties... Maybe you could cover Immortal or Burzum? Bathory?