I never really noticed much difference in the costumes between the movies. The armor pieces in ANH look a little more metallic where in the later films they are just black.

When you investigate where the name came from, it is almost impossible to ignore Dark Father and the implication that he was someone’s father from the very beginning. Whose may have changed from draft to draft, but I think by the time it got to the production phase that he was Luke’s father from the outset. Two clues give this away to me. First, the lightsabers. The Anakin and Vader sabers are nearly identical. This may be a coincidence because of the source of the parts, but Kenobi’s saber is vastly different. Second, Alec Guinness delivers a masterful bit of acting just before he launches into the explanation of Luke’s father. Either that is the most awesome coincidence or George told him Ben was lying to Luke. It is just a quick change of expression, but it reads to me like Ben knew what he was about to tell Luke was a deliberately slanted point of view.

Lucas and the Star Wars production were famous about not revealing secrets so Lucas could have had the existing story (of 4, 5, 6) planned out from the outset with Luke and Leia being siblings and Vader/Anakin as their father. He just didn’t tell anyone who didn’t need to know and even then, not before they needed to know it. Like Prowse’s line on set “No, Luke, Obi-wan killed your father.” Prowse didn’t know. Only Mark Hammil, Lucas, and Kershner knew on the day of shooting and they only told Mark shortly before shooting. Leia being Luke’s sister would be another such item that Lucas would not tell anyone the real story until it was time.

Also, the saber duel on the lava planet was planned from the beginning. I knew about it very early on. Not sure when it was mentioned or how. Though it was originally just a volcano.