You just made me picture Jordan Rudess doing the Gangnam Style dance. Curse you....
Myles Kennedy, Matt Bellamy, and Michael Eriksen joining James and the rest Of Dream Theater for one song would be epic. You've got Matt, who first got me into high male vocals, Myles, who has a great hard rock tone, along with fantastic range, and Michael, who, to my ears, is a mix of Myles and James Labrie, with a just a touch of fellow Norwegian vocalist Morten Harket of pop legends a-ha.
Imagine it, Blackbird meets the Exogenesis Symphony meets Architect of Fortune meets Metropolis Pt. 1 (four songs you should know (six if you count the symphony as three parts))
None. They could do some covers and invite people over like Russ Allen/the other guys for Cemetery Gates, but I wouldn't want guest singers on DT's album, because that would mean the song wouldn't be performed live (unless they'd plan on bringing that person out for just one song every night), which would be a shame if it were a good song. And I'd quite dislike a JLB/Sharon/Amy/Whoever duet, because Dream Theater is the type of band that... how do I put this... does things for the sake of doing things. They wouldn't write a song and conclude it would go well as a duet and then add a singer, they'd just write one straight away and I don't think that would turn out too well.
At the end of the day, This.