Black Sabbath "Black Sabbath" - I think that monument of hard rock and heavy metal need some growls. I've heard Vader cover, but it's not too dark. I think Opeth's atmosphere would be much more suitable to that dark mess.
King Crimson "Moonchild" - Mike's clean voice would sound pretty nice in the beginning as I think. And then, in the second part there could be some psychedelic jam - just like in live "Closure" version. And in the end, I think, Opeth could make some metal stuff with grunts. But, anyway, it's very opethian song, in my opinion.
Camel "Air Born" - I have no idea why Mike haven't some Camel cover still though it's one of his favourite bands. I think Opeth was influenced by Camel much bigger than by other 70's prog-bands. There could be vary songs Opeth should cover and "Air Born" is one of them.
Golden Earring "Are You Receicing Me?" - some classic proggy hard-rock stuff. Nice field for improvisation and Opeth's classic arrangements.
Judas Priest "Dreamer Deceiver" & "Deceiver" - of course, Mike couldn't do these Halford's falcettos, but... anyway, I think, he would find some idea to make it sound great. Very intime and emotional song for Opeth. In the heave part there could be some screams.
Led Zeppelin "Dazed and Confused" - I don't like LedZep, really, but they have some bunch of gorgeous songs anyway. And "Dazed and Confused" is one of the best - I think it has some fantastic balance of evil and emotions to fit in Opeth's style. I've heard some nice cover of it on "Stairway to Rock" tribute and there was a vocalist from P.O.D. or something... he've done some nice hysterical screams and it fitted perfectly. I think Mike could sing and growl it in his own style very nice.
Slayer "Seasons in the Abyss" - whoa, one of the most evil songs ever! Perfect for Opeth I think. Or it could be "Dead Skin Mask". Some Wiberg's keyboard magic and Aker's grunt would make 'em even better. "Dance with the dead in my dreams" - just imagine Mike growling these words!
Metallica "Harvester of Sorrow" - mid-tempo pounding classic! Also a great field for Mike's grunt and rich arrangements.
Whoa... there's shitload of such stuff. And "Billy Jean" is, of course, one of the main pretenders!