Neverar's idea gave me one of my own:
What if Anakin and Darth Vader were two separate pupils of Obi-Wan (this way we don't suspect Obi-Wan is lying when we get to ANH)? Darth would be a Sith with a mechanical limb or two, and he would battle Anakin at some point (the volcano battle). Anakin would be severely injured, having his lost limbs replaced by mechanical ones. His fall to the dark side would be partly due to his desire for revenge. The two of them meet again in a final battle on a starship and they duel. At one point, they force push each other into the vacuum of space and the scene cuts. The viewer doesn't know if either of them survived, and if so, which one, until a while later, at which point the survivor is wearing a black suit which resembles the Darth Vader suit we see in the OT. Palpatine (who would already be emperor at this point) turns out to be Darth Vader's master. What the audience wouldn't know (assuming they watched the movies in chronological order) is that Anakin made it back to the ship and was rescued by medics. Getting back to what the audience would see: this would be an imperial ship, so Anakin in the Darth Vader suit would be transported to see Emperor Palpatine. Palpatine would be angry at what he thinks is Darth Vader and torture him. Anakin, becoming angry, would fight back, nearly defeating Palpatine. But Palpatine would recover and eventually submit Anakin to his will.
The viewers would find out at some point that Darth Vader was merely the title of a Sith apprentice, and never find out the first Vader's true name. Whatever way Palpatine would use to make Anakin submit would turn Anakin into the next Darth Vader. The volcano battle would be in the second movie, while the other battle would be in the first half of the third. The remainder of the third would be Anakin continuing the first Darth Vader's mission to destroy all the Jedi (who would be scattered throughout the galaxy).
Of course, Obi-Wan's conversation with Luke in ROTJ would have to be different, with Obi-Wan saying something like this:
OBI-WAN: What I told you was true, from a certain point of view. Anakin fought and killed my apprentice, [insert name here] who was a darth vader. By doing so, he was consumed by the dark side and became the man he defeated--the next darth vader.
I really like Neverar's idea though, and think that that's what should have happened. If it is at all possible for Adywan to pull off something like that, he should do it. It would probably eliminate many other changes he would otherwise have to make to prevent the ESB surprise from being spoiled for a first time viewer of Star Wars.