I honestly don't know what could be added to the movies themselves. In a perfect world they would all be on one disc but that's not possible. Then you could have viewing options where you could watch them in chronological order, with or without deleted scenes where they should be, etc.

1. A book of some type, similair to what came with the VHS box set.

2. Include a DVD that had all the music.
3. More behind the scenes footage.
4. Modern interviews with the actors.
5. talk show appearences made during the promotion of the movies.