I believe that Vader was going to be someone’s father just from his name (a Germanic version of father), but we have no concrete idea who that was going to be. There are no clues in the drafts of the original screenplay.
Darth Vader comes from “Dark Invader/ Death Invader”. It shows Lucas’ original notes in the making of ESB book and states that “Lucas’s notes that led up to the name of his villain: “Dark” and “Death” became “Darth,” and “Invader” was shortened to “Vader”—Darth Vader.” It’s just pure coincidence that Vader became the Father and the German “Vater” meaning father, are similar.