If Vader had not been Luke's father, what would the plot of the Original / Prequel / Sequel Trilogy have been? I think in this alternate scenario, it could be said that Obi-Wan was being literal when he talked about Luke's father, i.e, Anakin and Vader are virtually two different men.
Luke Skywalker: How did my father die?
Ben Obi-Wan Kenobi: A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi knights. He betrayed and murdered your father. Now the Jedi are all but extinct. Vader was seduced by the dark side of the Force.
So the prequel trilogy would have shown that Vader was a mate or friend of Anakin Skywalker, before betraying him.
Let me hear your thoughts about this "what if" scenario
Plus, and just for fun