I'd love to see the "Next on..." trailers, galleries of artwork and storyboards, and any and all deleted scenes. Some thoughts below...
-Text commentaries (I think the Okudas have done some for the various compilations)... I see us getting anywhere between three and six per season, though every episode would be nice (the Seinfeld DVDs have them on every episode)
-Audio commentaries (again, I think a few were recorded for the compilations)... I doubt anyone from the cast will sit in for one, other than maybe Frakes, Sirtis, and Wheaton... but I can see the Okudas, Dan Curry, Ron Moore, Brannon Braga, and the other writers doing a few, along with some of the regular directors and maybe Trek experts like Jeff Bond and Larry Nemecek... as for recording them for every episode... not a chance!
-They'll probably include the Best Buy-exclusive featurettes along with the three new featurettes that were produced for the TNG Complete Series set
-I'd love to see new interviews with the "TNG stock company" (regular guest stars)
-Tributes to Michael Piller and Majel Barrett would be much appreciated!
-I doubt we'll ever get gag reels (why start now?) or isolated scores (Film Score Monthly's Ron Jones boxset is awesome!)