Vader wasn’t Luke’s father at first. In Leigh Brackett’s first script draft, Luke meets his real dad, who says he sent away Luke and his secret sister for their own safety. (Luke’s sister has been training to be a Jedi knight in secret, just as Luke has.) And Papa Skywalker administers the oath of a Jedi Knight to Luke, in which Ben, Minch, Anakin and Luke cross lightsabers, and Luke swears to “dedicate my life to the cause of freedom and justice.”
Reading through these, I can't imagine the movie being that good if these choices had been made. There's also some really cool concept art from the movie, so you'll definitely want to check it out. If you're a Star Wars fan of course.