Also, I agree that both Yoda's fate and Leia's parentage should be up in the air until the Original Trilogy. Maybe as a squadron of soldiers are bearing down on them, Yoda hands Luke to Kenobi and says something like, "Our only hope this one may be. Safe keep him you will. More fight do I still have this day." Then he can go jumping into battle whatever is coming with Amadala screaming giving birth to Leia in the background. That would preserve the shock of seeing Yoda alive and Leia as Luke's sister.
Rebel11_38, the only way to write Episode 3 to make it so the audience doesn't know Anakin is Vader is to give Kenobi another apprentice. You can't have Anakin with "Sith Eyes", can't have the Anakin-Kenobi duel, or show Anakin alive after falling into the lava. You would have to have Anakin duel the other apprentice. The other apprentice would win and escape. He would have either never be seen again, run into the Emperor, or meet with a fate that could either kill him or force him into the suit. Kenobi will either think Vader is the other apprentice or force sense Vader is Anakin. Although, in order for this to really have worked as a movie, the other apprentice would have needed to be introduced in Episode 2.