Here's a few that I think DT could really do justice to:
Metallica--Unforgiven III: I'd love to hear it, just to see JLB exceed Hetfield's scream.
Foreigner--Jukebox Hero: While you can't really match the original, this has a great epic feel to it that DT could replicate quite nicely
Rush--BIg Money: I have no idea how this would sound, but I think this would be a cool slant on their 80's synth-era material
Jesus Christ Superstar--Gethsemane: If JBL is on his game, the vox on this would be truly amazing
Scorpions--Woman: A lesser known tune from the Face the Heat album, this would be really smoking
Nine Inch Nails--Sanctified: JM's bass riff on this one would be incredible.
Judas Priest--Touch of Evil: While no one can touch Halford, James screaming "YOU'RE POSSESSING ME!!" would be out of this would