As for Vader's motivation: Lucas suggested that Vader knows he isn't powerful enough to turn Luke to the dark side on his own. Thus, he wants the Emperor to do it--at which point (Vader thinks) he and Luke can unite to overthrow the Emperor and take over the Empire themselves. (Unbeknownst to Vader, the Emperor of course has other plans.) This is a motivation much more consistent with Vader in ESB.
Lucas immediately hamstrung this interesting idea, however, by suggesting that it should never be stated outright, but only hinted at obliquely. Otherwise, he thought it would telegraph the fact that Vader is the one to kill the Emperor at the end of the film.
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Lucas' initial idea in regards to the Imperial capitol planet--Had Abbadon--was that the Rebels should capture the Death Star being built in orbit, use it to destroy Had Abbadon, and then destroy the Death Star itself. But he came to regard this as too cumbersome a plot, and found having the Rebel fleet itself bomb Had Abbadon to smithereens visually dissatisfying. (He wanted a single gigantic explosion to cap the film.)
The problem seems to have been that Lucas was leery of having a massive city as a major set-piece of the film, especially in terms of showing the massive (and no doubt expensive) air and ground battle required to destroy it. Instead he was much keener on a battle in the forest, with the primitive Ewoks overcoming the Empire. As he puts it "I wasn't very successful in coming up with another idea of how to finish it off and still keep my Ewoks going."
Kasdan on the other hand was much keener to keep Had Abaddon in the picture; he liked that it was visually and plot-wise different from the Death Star of the first film.
Kasdan suggested that Luke should take over the Empire at the end of the film, becoming the new monarch of the galaxy. Lucas on the other hand rejected this suggestion, insisting that the Republic would have to be restored, and that Luke wouldn't want political power.
There was a lot of discussion of Darth Vader's appearance; Kasdan encouraged Lucas not to "wimp out" and make Vader too normal-looking; Lucas on the other hand suggested that the glimpse of Vader seen in ESB was too over-the-top and scarred for his liking. They did agree that Vader might have only one good eye, and perhaps a gray beard to better resemble Ben Kenobi.
Lucas was already pondering the possibility of cutting away during the final celebration scene to scenes on other planets. He rejected it at that time, considering it more proper to keep the ending small-scale.