I certainly think that Lucas should release the original versions of the trilogy as maybe a "Classic" collection, but he also should continue to improve the SE/DVD version.
First off, John Williams really should rescore and re-record a lot of the score with the much larger orchestra he has used since the Prequels. He can then add in the Imperial March and any other themes he wants to add. The crawl was redone very well for the DVD release, so it just needs re-recorded music.
His has numbers at the bottom which bug me, while Darth Maul's had an alien language.
The film isn't about Vader(to the extent ESB was) so Williams gave him a rather low-key theme, adding the bombast of the IM would completely change the feel of the film, and be a huge mistake IMO.
Actually Lucas has said several times that the whole saga is about Darth Vader.
To deny him his theme in the first movie we see of him since the climax of Episode III would just be a crime.
And besides, if GL did release the original versions on DVD, why would anyone who liked the originals really care if he made major changes to an alternate version?