I would have loved a Warrel Dane version of "Hushabye Mountain", preferably arranged by Danny Elfman. As Warrel is no longer with us, I think Terrance Zdunich might fit such a creepy arrangement pretty well.
I would have liked to hear a cover version of "time to say goodbye" with Freddie Mercury on the male part. However, I would keep Sara Brightman as the female counter-part in this song because I like her youthful/light tone so much in the song that I think that no other female singer would have done this song justice.
I'd have loved to hear a prime Ted Neeley sing 'Children of the Damned' by Iron Maiden. Also, a WASP/Blackie Lawless version of 'Merry Xmas Everybody' by Slade would be good fun. Finally, if it hasn't already been suggested, a Halford 'It's Raining Men' in his gritty Painkiller-era style.
This is ridiculous, who wants to pitch a backing vocal B2 at three in the morning? OH BOY 3AM!