The Ben v Vader duel in ANH: it can be improved with the existing footage and if needed a few new shots could have been done with doubles to make this fight more nervous.
If another change could be wished, I would go with new scenes for the battle of Endor to see a real fight between ewoks and stormtroopers (and something close to that could be reinserted: https://www.youtube.com/watch?v=gT-ZsR2P8dU).