A couple of thoughts on Luke and Leia:
-Don't limit yourself to making them siblings. Since you're rewriting V and VI as well, why not utilize the original plan of Luke's sister being hidden on the other saide of hte galaxy? Though I can see the disadvantages to this in only 6 episodes, with her showing up "at the last minute" not allowing for any character development.
-If they're not siblings, they can end up together in the end, something I would have liked to see happen. With Han either dying or him and Leia deciding they're not for each other. But again, with only 6 episodes this can make Leia seem less than honorable with her flip flopping between them.
Just a couple of ideas Lucas threw to the wayside that I planned to use in my versions.