Well... That was kind of my point... I don't think the O.T. really needs prequels. Not directly related to them anyway. I makes more sense to me to keep the Star Wras Trilogy as, just that, THE trilogy. You're not bogged down by having to tie everything up and dealing with expectations from fans that way. Stand alone films could tell amazing stories and keep the feel of the original movie a lot more.
If Darth Vader's backstory has to be told however, here are a few changes I would make.
Firstly, the prophecy should be axed. It is never fully resolved, mainly because Lucas didn't know he would make prequels when he did ROTJ.
I also think Yoda needs to go... Not just the lightsaber duels, everything!!! He is mentioned by Obi Wan, such as when he's training Anakin and after Anakin's fall to the dark side. But other than that he doesn't feature at all until Empire. It's just better pay off for the character.
As for the whole "I am your father!! Bit... The best way, in my mind, to keep this a surprise is to have it seem as though Anakin dies when Obi Wan defeats him on Mustafa. The emperor and his storm troopers could find his charred remains and say something like "there's no way he'll survive injuries like that..." Or something!! Lol
Also, Anakin is never named Darth Vader, you see him killing some of the Jedi and you know he's become evil, but his name doesn't change and you assume he's dead by the end of episode III. Darth Vader emerges as the Jedi hunter right toward the end of the film, but we still don't actually see him. Maybe we see Obi Wan in hiding and he overhears a news report, or even people talking about "the Emperor's new bulldog hunting down more jedi..." That's the first time we hear his name but we never see him in the armour in the P.T. That way, when he enters Leia's ship at the beginning of STAR WARS we're still not 100% sure who he is.
Well... Those are a few thoughts anyway... I have more if anyone's interested in hearing them... :)