Your description of ROTJ as 'Star Wars: The Greatist Hits' is spot on Bingo, it would be great to break the formula that Lucas uses (although 'last movie there was one, this movie there'll be two!' falls into his film making formula also).

Adding a second Death Star would radically change the movie, but how could th rest of the movie tie together, new elements would need filmed including Ackbar & Lando's reaction. Will there be a decision for a second DS attack run, if so would this not be confusing for the viewer, Lando's DS Attack run, Luke's battle in the tower, Han & Leia's Endor adventure with the Ewoks, then _____?'s attack on the other Death Star.

Maybe thats why Lucas dropped the 2 DS scenario.