Personally, I absolutely love the prog adaptations of classical works.
I think it was ELP's "The Barbarian", off their very first album that
got me really hooked on the idea.
Out of the classical pieces I'd like to see done in prog style, there's a few fairly obscure ones, namely the
Sederunt principes by the 12th Century French composer Perotin. It just absolutely rocks.
I think that there's some prime candidates in Darius Milhaud's string quartets as well, namely the second movement of Quartet No. 2 or the third movement of Quartet No. 4.
And of course, since King Crimson and Emerson Lake and Powell covered "Mars" from Holst's
The Planets, why not cover some of the other movements? I think Uranus would work particularly well. Bartok's stuff generally seems to be quite conducive as well--particularly the fifth movement of his Fourth String Quartet.
I could go on and on, but I'll spare you.
-Topographic