There are a number or things I'd do differently. Instead of a new Death Star I'd come up with a new type of Star Destroyer, larger than a Super Star Destroyer, like this one:
http://starwars.wikia.com/wiki/Eclipse-class_Super_Star_Destroyer
And I probably wouldn't have cast Ian McDiarmid as the Emperor. Don't get me wrong, I liked his performance in ROTJ (less so in the PT, but he still had a bit of charm there), but I would have taken Palpatine's character in a whole different direction.
The dark side is supposed to be seductive, yet Lucas never went and made any dark siders seductive themselves; they all looked old, or deformed, or stereotypically Satanic. I would have done the opposite. I would have cast a woman as Palpatine - preferably young and at least moderately attractive - and brought back Clive Revill to do Palpatine's voice. Instead of snarky and sarcastic, Palpatine would sound calm, sauve, and at times even genuinely remorseful. He would be this great dark father - mother? - figure.
I probably would have also replaced the Ewoks with Wookiees, or some other non-dwarfish race.