I've had a bunch of different thoughts on this that I'll try to separate by post:
So much of the glimpses and hints of the back story we saw prior to the making of the prequel films seems so intriguing even still a dozen years later.
I think personally the two key elements are Obi-Wan and Anakin. Its really their story, and in an odd way serve as the eventual respective "parents" of sorts to Luke. (This also helps to indicate the difficulty in writing about Anakin's wife given that she's an entirely self contained character in the prequels, but that's another matter entirely) Serving to how they ended up the way they did, and the falls of the two men (yes both) are what would make the story such a great tragedy.
A lot of my thinking is going back to the various novelizations of the films, which do serve to have some interesting gems as far as character and plot points, especially with Kenobi in ROTJ. Indeed, having read that, I'm thinking there could be great potential in making Kenobi the one who's too sure of himself, too confident, too convinced he's the best possible teacher of Anakin. Obi-Wan himself is a generally good person, and too dedicated to his morals and ideals to turn, but his own massive character flaw does help to doom his student.
I mean, we see in the original trilogy, an all knowing, well traveled mentor, who seems to possess wisdom beyond anything we could imagine. That sort of character, to me, feels like it means so much more if that wisdom came at a costly price. Truly understanding that much of Kenobi's actions in the original trilogy are to make up for the arrogance he had and resulting failure in him being a mentor and teacher to his student makes those scenes with Luke mean all the more. We see why he is a better teacher now, than he was then.
Which of course, brings us to Anakin, arguably the real lynch pin of the entire prequel films. Something that I always wanted to witness in the first three films, is not simply how Skywalker turned, but how he turned into the exact kind of cold hearted merciless Sith he was the original trilogy. Evil can have many personalities, and the Vader of ROTS seems to resemble the one from the originals only cosmetically. He's an evil guy in a suit, and that seemed to be as far as they went with it.
Its why I believe its critical to see exactly what character traits of Anakin's that either through being twisted or exaggerated, still existed within Vader. With that in mind, Anakin would have been better off as, well, being genuinely tragic. Somebody who starts out much like his son does; an eager and optimistic young man out to take on the world. Yet that while conflict and problems allowed Luke to grow into a mature, but still visionary member of the Rebel Alliance, his father grows into an increasingly shell shocked and bitter man.
Indeed, both Luke and Anakin start off very similar (with perhaps the former at bit more inclined towards seeking adventure). The key split is that Luke never lost his idealism. With the support of his friends, the guidance of Yoda/Kenobi and the unfortunate example of his father set, Luke avoided it.
Anakin wasn't so lucky. The sense of patience and discipline that defines him early on start to disappear (with him slowly getting less and less forgiving of mistakes and failures of those under his command) as the battles take a toll on him. Skywalker, a young man who started out on the "damn fool idealistic crusade to make the galaxy free, just, and peaceful becomes a desperate individual tired of the death and destruction of the Clone Wars, and is eventually just looking for peace in the galaxy at any cost.
Enter Palpatine who's able to offer him just that. The dark side being quicker and easier simply is far more appealing and turns into what ultimately drives him to trust Palpatine's judgement. Skywalker seeing the Emperor's rule as a fair price to pay to potentially save millions of lives from the war becomes the major moral compromise that ultimately dooms him. More importantly though, it helps to explain the specific facets of Vader in the original films. Its why he's so unforgiving of his own officers, why he's so personally determined to crush the rebellion, and why he made the offer to Luke in ESB.
That desire for the galaxy to be safe, the thing that made Anakin into a Jedi, is what serves to damn him so badly, because after that one compromise with Palapatine, everything became easy to justify. Imperial officers ruining impeding his plans, the Rebel Alliance causing war, his son possibly opposing him; they're all hindrances to peace, all the things preventing him from indeed "bring(ing) order to the galaxy". It takes the cruel sadism of the Emperor's torture of Luke, and recognizing the true goodness of his son to make Vader realize not only that he himself needed to be saved, but that he could be saved.