Now, this is all implying Lucas’ vision of the PT. It could have been set from an entirely new character’s adventures / struggles and not Anakin, in which case maybe he could have showed up in the first film and then been seemingly killed. But as long as the trilogy is about Anakin’s fall or Obi-Wan’s failure, I don’t think you can thematically justify keeping the plot twist.
Well, yeah, I guess in my hypothetical PT it wouldn’t have been about Anakin’s fall any more than the OT is about Lando reforming himself as an honorable man. Anakin would have been the equivalent of Han in the first two (if Han had died on Bespin), with Obi-Wan being the clear protagonist (keeping with the “trio” idea, Obi-Wan = Luke, Anakin = Han, Luke’s mom = Leia).
But that’s getting pretty off topic for a “change one thing about each movie” thread. 😉