I like the idea of the whole finale of ROTJ being an assasination, and a second death star is pretty lame. Maybe you could have the final Vader/Emperor/Luke battle in a windowless room to eliminate the need to composite coruscant outside. As somebody has already said, watching a space battle miles overhead from a window in a room in a city is impossible anyway. Perhaps the Emperor could help Luke to watch the battle via the force (implanting images/allowing Luke to 'see' through him). How the hell you would make it clear what is happening is beyond me though - Just throwing some ideas out.