JimmySilverrims

I think both of you are right.

Lemonstein, It's quite obvious that the Prequel Trilogy is unique and different from the Original Trilogy, with new characters, locations, and plot that tells of a story that is very different than the original trilogy.

However, as Bingowings points out, this is not two trilogies, but a saga. Taking place in the same universe, with one story pulling it towards aa conclusion.

So by making the identity of Vader intentionally vague (without eliminating the fall of Anakin [He still turns to the Dark Side, just {seemingly} briefly]) we keep the Prequel trilogy valid while still preserving the ESB twist.

Anakin's fall is intact, the rise of Palpatine is intact, and the saga (as many of us so desire) becomes more streamlined, feeling less like an original and a prequel and more like one long story.

Also, the Emperor in TFU is Sam Witwer, the actor for Starkiller. I was surprised at how good his impersonation was.

Also additionally, the Executor scene would have to be the last scene of Episode three, making it appear as though this takes place sometime in the future and this is the reveal of Vader.

I'VE GOT IT!

There is one other option.

A simple, elegant solution that keeps the twist in ESB a surprise, but also keeps Anakin's slip to the dark side. It would be easy to do and be non-intrusive to the story. But it isn't perfect (as virtually any change this large would be.)

Make it appear Anakin dies at Mustafar. The original scripted line in ESB was not "I am your father" but rather "Obi-Wan killed your father". If you make it appear that Vader is about to tell Luke this instead the surprise is in tact.

Add more flames to Anakin's burning, and remove all shots of him moving (the scene in which the Emperor reaches down to touch him is excellent for this purpose as he does not move at all, save for his head turns slightly.) Remove breathing and Palpatine's line "He's still alive".

Instead, have Palpatine say "I must make a new apprentice" (Likely using a voice actor, as any line in which he says the phrase "new apprentice" is over hologram. Every. Last. One.)

Likely, (and most regretfully,) you must cut most if not all of Vader's birth. (Which blows, because that was one of the most redeemable scenes in the film. [Except for that last bit "NOOOOOOOOOOOOOOO!"]) Perhaps you can make it appear as though Vader's a droid or something, but most likely just have him appear at the scene with the Emperor.

And I know that it might seem out of place for a Star Wars film, but preface the Executor scene with text saying "Ten Years Later". The idea of the Emperor building a new ship and handing out new uniforms so quickly seems ridiculous, plus it makes it look like it takes less time for the Death Star to be built (which was another major complaint I've heard.)

It appears that Anakin is dead, Palpatine has a new apprentice, and an explanation as to why Obi-Wan would say Vader killed Anakin.

The only issue? The excellent lines "he was a pupil of mine, until he turned to evil" and "when I left you, I was but the learner, now I am the master" must be sadly cut from ANH.

Otherwise, this seems to be the most simple, logical, and viable solution to keeping Vader's true identity a secret.

So interesting!

I had heard the Maul/Grevious ideas before, but this actually isn't very bad. It adds depth, keeps a great character, and on paper, it's very cool, but it does raise some problems:

Why would Maul be a general if he was the bad-ass secret sith assassin from TPM? He was more of the quiet follower rather than a military commander.

Why would Palpatine spend so much time with Anakin? Why would he get Anakin to kill Dooku? 

How do you make a reveal of Maul's resurrection? How do you hint to it throughout the film? How do you depict his second resurrection?

How do you convey that Vader and Grevious are one and the same when Grevious is spindly with no organs and much machinery, while Vader is clearly humanoid?

How do you include Anakin's "death" or fight with Obi-Wan without making it obvious that he's Vader?

Should you have Obi-Wan recognize Maul? If not, how to you interestingly convey who he is?

Why would Palpatine sink so much effort into keeping Maul if he has already replaced him with Dooku?

What do you do with Obi-Wan's fight with Anakin? What causes Padme's death? Does Anakin still cut the hands off of Windu? Does Windu confront Palpatine at all? If not, how does Palpatine become disfigured?

How much new footage will it take to add the Maul Resurrections story arc? how do you make it seem like "Maul"'s body was recovered from Utapau without actually showing it happen (as it actually didn't).

Now I know that sound like a barrage of negativity, but really I'm just wondering how on earth anyone could pull of such a massive change without anything short of inserting entire new sequences into the film (a feat that even funded professionals would have time with) and add in lines (making the whole attempt damn near impossible).

I say near impossible. If you made Maul speak a subtitled alien language in TPM and if you had Grevious speak the same language the same way and if you managed to film new sequences of Grevious's reveal in AoTC and his resurrection in TPM you could (with difficulty) make me believe that Maul = Grevious.

But Grevious = Vader?

Bingowings said:

What about merging Utapau with Mustafar?

Obi-Wan is officially there to bring down Grievous who falls to an uncertain end in a lava pit, Anakin is unofficially there to kill the Separatist leaders (thinking Obi-Wan) already dead.

That way you could blur the identity of Vader by insinuating it might be Grievous reborn and still have Anakin (not yet Vader ) fight Obi-Wan because he is a threat to the Chancellor and have a suited Vader enter the Temple and dubbed Lord Vader.

That's not a bad idea but how could that possibly be done? All introduction shots of Utapau would have to be replaced with entering Mustafar (inserting Obi-Wan's fighter, I suppose). All of the scenes of Obi-Wan on Utapau would have to either be heavily edited (and would be a very obvious edit) or, more likely, be completely unusable.

Also, how do you make it seem that Grevious meant anything to Palpatine, or at least enough to bring him back to life (apparently for a second time)?

Not that there isn't a possibler solution out there...

I'm not going to lie and said that I've meticulously analyzed this gargantuan thread, but I have read quite a bit of the most popular suggestions, and I have one great concern:

Adywan has openly voiced his desire to keep the reveal of Vader as a surprise. This is logical, as the twist in ESB is essentially negated by the prequel trilogy, and would be much more enjoyable if it was kept as the amazing twist it was intended to be.

This preservation however, raises some problems that no one has proposed viable solutions to. How do you have the prequel trilogy, in which the ENTIRE PLOT centers around Anakin's inevitable downfall, stand without it's core motivation? And how would you even make it even slightly ambiguous without butchering the ending?

To keep Anakin's transformation secret, you must:

A.) Make the audience believe that this Vader is a new character without seeming out of place

B.) Explain how Anakin becomes Vader without it being blatant, but also holding up to logic

C.) Make it seem like these two are not one and the same, and

D.) Make it appear as though Vader has killed Anakin.

I have yet to read a single viable way of making any of those elements work. This is a major flaw, several major flaws actually. Enough flaws to make me question as to whether keeping the secret would be worth it or not.