Alright, here is my attempt at an answer to the OP, and to Puggo.
I think preserving the OT's surprizes was worth doing, and I think it was doable. Moreover, I think that had it been done properly, the way it was (un)handled in the PT could have actually enhanced the surprizes in the OT.
For example, the Vader->Anakin bit was surprizing in 1980 because of its implications to Luke. Anakin was persona incognita for the most part. If Anakin is well known due to the PT, and Luke is well known due to 2/3 of the OT, then the surprize isn't just "Oh noes! Luke is the son of a bad guy!" it's also "Holy capr, Anakin is still alive and has been Darth Vader this entire time! He's even more evil than I previously knew or thought!"
But let's take a step back for a moment. I'm a proponent of a Mulligan Prequel Trilogy. If that topic interests you and you haven't been to the rewrite portion of this site, please step into my office (er... this forum where I post a lot.) It can be found here: http://originaltrilogy.com/forum/forum.cfm/Script-Writing-and-Re-writing/forum/21/
My comments will not be based on a "what can a clever film editor do?" so much as "What could Lucas have done in the first place?" and "What could a Mulligan PT do now?"
Whether the proverbial genie (surprizes) can ever be put back in the bottle (not such a prevalent part of pop culture that anyone watching the OT, even for the first time, probably already knows all of the twists and turns) I think is a worthwhile subject for discussion, but I think the answer won't change my position. It's just sort of the art of the thing. Whether episodes 1-3 are made after 4-6 and what the intended viewing order is supposed to be... that doesn't matter to me so much as the artistry of pulling the whole thing off. Everyone walking into your standard RomCom knows the cute couple will overcome their obstacles and get together in the end... but that doesn't mean that you don't try to drum up the drama anyways. Maybe an ill fit comparison, but I think at least some of you will know what I mean.
As I see it, there are three OT surprizes to be preserved.
Vader is Anakin
Leia is Anakin's child
Yoda is a little green muppet, not some great Jedi warrior
1. "The big one" AKA Vader is Anakin. Short synopsis of what the PT shows. Obi-Wan trains Anakin, against his better judgment, to become a Jedi. Anakin turns to the dark side. Obi-Wan is left with no alternative: They fight- Obi-Wan wins. ANAKIN DIES. It's sad, the end.
The first thing you'll notice is the impact this has on the OT. We see Vader whom we don't suspect to be Anakin (more on that later). We see Luke, whom we know to be Anakin's and his wife/mistress/girlfriend/neigbour's kid that was born after Anakin's death. Luke's not with his mother, so we might assume that something has happened to her as well, but we don't know. The droids introduce Luke to the name/idea of Kenobi, whom we know from the PT as the good guy who made a mistake training Anakin and as a result was forced to kill his friend. Luke meets him and we're surprized at how old he has become (and what's with the hermit robes?) but whatever- He begins to tell Luke the good parts about his relationship with Anakin, and we wonder if he'll tell Luke the hard truth. He doesn't, but in order to avoid the truth, he has to lie to Luke about who killed his father. We know Obi-Wan killed him, but he lies and says it was Vader. We don't judge Obi-Wan for this lie- what else would you tell an impressionable young man?- but we suspect that this info might come back to bite him later. We see Vader and Obi-Wan meet/fight, there is dialogue about their last fight (more on that later). Next movie, Vader is pursuing "the son of Skywalker", they eventually fight. And we, as an audience, are prepared for this:
VADER: Obi-Wan never told you what happened to your father!
LUKE : He told me enough! He told me you killed him.
VADER: No, Luke. HE killed your father!
LUKE : No, Obi-Wan wouldn't have...
VADER: It's true, Luke. Come with me if you want to learn the truth about the Jedi!
LUKE : I won't!
VADER: Not today, but you will. And I'll be waiting.
We know Anakin turned to the dark side, so we have this constant fear that Luke might as well. We know about Obi-Wan's lie, and we worried about how that might be leveraged against Luke. But we never see this coming:
LUKE : He told me enough! He told me you killed him.
VADER: No, Luke. I am your father!
As I mentioned earlier, the implications to Luke are still there- still surprizing. However, we get this new sense- Anakin, that tragic figure that we loved and lost in the PT, we get him back! We thought we had a small piece of him- the continuation of his story in Luke, his son. But now we have so much more. This Vader character whom we thought to not have much attachment to, we now realize is our fallen hero from PT 1-3. Without the PT, we only have the feelings for Anakin that a son raised with only stories of his deceased father might have. But, this scene from the perspective of the (correctly done) PT... is so much more. We might have believed that Anakin was simply teetering on the edge of the dark side and that, given the chance, he would have come back to his senses. We might have told ourselves lots of things about Anakin and what might have been. But now, all hypotheticals are thrown out, because here he is. And he was right under our noses. He is not dead, but he is not doing well. Like Luke, our purposes now are to see him returned to the light. However, unlike Luke, we know the power of the dark side and we fear that we might lose him too. This makes our fears and hopes going into RotJ all the more real. We lost Anakin once. Perhaps we can get him back. Perhaps we will lose Luke the same way.
So, that's all great for the OT... but how do we pull this off in the (m)PT? Well, here we have a couple of options. The PT needs villains. Anakin certain becomes one by the end, and we have to assume that the man who would be Emperor will be one as well. Then there's something about an enemy army and whatnot. But surely, there must be some mold into which we can fit Vader. I'm not saying it has to be a Darth "somebody", but I won't say it can't. But there has to be something that manages any suspicion from Anakin's death in Episode 3 cutting to Vader's appearance at the start of Episode 4. And then there's that business of what to do with Obi-Wan's story about "a pupil who turned to evil, became Vader and betrayed and murdered Anakin". We probably shouldn't have Anakin be Obi-Wan's only apprentice, or else that lie might start to wear thin(ner).
Inset: Someone wise once said here, "So your mPT now features an Obi-Wan with 2 apprentices who both turn to evil? How bad of an instructor is he?!?" This is a good point. A possible solution is to ensure that Obi-Wan has many whom one could call pupils. He's no master, but perhaps there are many around him that are on their own path of progression through the force who look to him as a mentor or tutor. This not only brings Obi-Wan's batting average up a little, but it introduces a bit of mystery as to who the bad pupil(s) will be, as opposed to you already knowing if you've already seen the OT.
So we now have another apprentice who is the likely candidate for becoming Vader. Since many (not I) wanted to see Anakin "put on the suit" in the PT, perhaps you go right ahead and put this guy in the suit. Or maybe just "a" suit. Does this guy get to be Vader before Anakin? What if he does? Is this a sacrilege? I suppose many of you will feel that it is. But let's run with it for a little bit. At the final battle, Anakin duels Vader(the first) and mortally wounds him. Obi-Wan shows up to help, but Anakin is fighting with so much rage and anger that Obi-Wan realizes he must save his friend from himself more than from any attacks from this Vader. This attempt leads to their own duel. Mortally wounded Vader sneaks out the back to go and heal his wounds. Obi-Wan's and Anakin's fight heats up (ba-dum-tish!) and Anakin takes a lava-nap. A heartbroken Obi-Wan leaves the scene, taking the fallen hero's lightsabre to the man's pregnant wife, "notPadme". She anticipates bad news- she knew that Anakin was slipping to the dark side- so she's very anxious when Obi-Wan returns alone. Obi-Wan silently hands her the lightsabre- she asks him what happened. Maybe the lie starts here:
OBI-WAN: He was fighting Vader. I rushed to his side, but I was too late. Vader escaped. He can run, but he can't hide. We will have justice. But take this lightsabre and save it for your son. Anakin, who died a redeemed man, would have wanted it.
This lie doesn't just protect Obi-Wan from accusations of wrong doing... it also honours the memory of his friend. Saying he died a hero instead of a villain. It's a lie, but it hurts much less than the truth. It protects notPadme's feelings and frees her from concern about the child she conceived with Anakin.
Or, perhaps he's totally honest with her. Perhaps the two of them come up with the lie. Perhaps we don't know about the lie until ANH. Your choice.
Okay I think that's basically it. Maybe there's more that I'm forgetting. I'm sure no one will like it (okay, maybe one other person will like it). Maybe people can offer some tweaks/completely different ways of doing it. (others will offer thoughtful "tl;dr" or "wall of text lol" comments) But that's one way.
#2 and #3 are relatively short.
#2 notPadme is pregnant with Anakin's child. Anakin is able to sense Luke through the Force. No one knows (in a world of mud huts, spaceships, and no sonograms) that there are actually twins. Perhaps there is some connection between notPadme and Alderaan (she's an Alderaanian princess/she's betrothed to an Alderaanian prince) but since the child(ren) is/are born after Episode 3 ends, then Luke is proudly presented as the result of that pregnancy. No questions asked, no muss no fuss. Again, we don't know what happened to notPadme (yet) but we're not overly curious. It could have been anything. DONE.
#3. There are two ways of handling this. The "never mention Yoda" way, which totally preserves the surprize. Or the "mention but never show, or go into too many details about Yoda" way. Yoda is not a warrior. Obi-Wan Kenobi was a general in the Clone Wars. Would they necessarily be in the same place at the same time? I say: no. Would Obi-Wan train Anakin himself, against the known wishes of his Jedi superiors, if Yoda was there and could have trained him? I say: no. Can Obi-Wan still drop the occasional "I wish Yoda was here!" or "I wonder what Yoda would do in this circumstance?" I say: absolutely. If all of the Jedi in the mPT are all powerful "Jedi Warriors", then the suspicion of a little green muppet being the greatest of Jedi is even lower. Yoda has nothing to do in the PT, so not being able to use him is no great loss. Keeping him out has potential for gains. Therefore: keep him out. QED.
I think someone suggested that Yoda was a surprize because theretofore the only Jedi had been human- Ben, Luke's father, Vader (from a certain point of view), and Luke himself. To the degree that other alien species are shown as aliens in an mPT, it weakens this. But to the degree that they are shown to be physically impressive in addition to their power in the Force... I think you'd be okay.
THE END.