Let's think outside the box on this one...

[TAC]

Yeah, Blood On Blood is a great song. it always reminded me of Thin Lizzy's It's Getting Dangerous from their Renegade album. (check it if you don't know it!)

As far as Priest goes..can you imagine James singing Riding On The Wind?! That'd be insane!

[El JoNNo]

Oh oh Judas Priest - Beyond the realms of Death   

[CrimsonE]

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

[contest_sanity]

Bryan Adams - "Run To You"
Billy Joel - "All For Leyna" (a fun song to let Jordan loose on)
The Beatles - "While My Guitar Gently Weeps"
Ozzy - "Perry Mason"
Van Halen - "Don't Tell Me (What Love Can Do)"
Journey - "Wheel In The Sky"
Megadeth - "Holy Wars... The Punishment Due"