NeverarGreat said:

Anakin is crushed beneath the weight of his failure and becomes a shadow of his former self, and takes his place at the Emperor's side.

This is something that I always felt was missing (among many other things!) in the PT. Even as a child, I was always moved by Vader's line in ROTJ: "It's too late for me, son." We never got to see any remorse from Anakin in the PT, unless you count the horrible "NOOO!". I think there was also a whiny and insincere "What have I done?" when Wingless Windu took his dive out the window.

You could convey this remorse and make the audience think that Anakin dies at the end of Ep.III to still preserve the reveal. Presumably dies in battle or duel? Have it happen off-screen: Obi-Wan tells Yoda/Padme later "I sense a disturbance in the Force. I believe Anakin is gone."