Anyway, here's a few ideas I have had:
1. The Empire. As I said in the aforementioned topic, in my prequels it would already exist. There would be no "origin of the empire" story (mostly because I thought about it, and realized the one given in the A New Hope novelization works as good as anything).
2. Darth Vader would be in all the prequels. BUT! The Vader in the first two would NOT be Anakin, but rather would be a human who has been turned into a cyborg, made from a highly-skilled bounty huntress (yes, a female Darth Vader). This Vader would be destroyed by the Jedi, only to be replaced in the third movie by an even more advanced Darth Vader who horrifies everyone because he fights like a Jedi and has control over the force! By the end of the movie, Obi-Wan would have figured out who this new Vader is, though he would not explicitly state as much. Which brings me to...
3. Anakin: Instead of a love-story/manipulation plot, Anakin Skywalker would simply be a pupil of Obi-Wan's who ignores the "peace/self defense" side of the Force and falls in love totally with the power aspect of it. In the second movie, Anakin would attempt to single-handedly take on the empire, having delusions of godhood. He'd put up an impressive show--even making Star Destroyers implode (after all, he DOES say "the power to destroy a planet is insignifigant next to the power of the Force") but a more powerful force from an unseen entity would fell him. Then, a voice would be heard commanding an imperial troop to bring him to the Emperor's ship.
4. Recurring characters from the OT: Only Obi-Wan, Anakin/Darth Vader, and maybe R2D2 and C-3PO would reappear. Other OT characters who would realistically exist at this time would never be seen (especially Yoda--remember, his first appearance was supposed to be a surprise too!) and the rest of the cast would be entirely new characters (also, absolutely NONE of them would be holdovers from George Lucas' prequels--no Quigon, no Jar-Jar, no Padme, etc.) My idea of a (sub)plot for the third movie would involve Obi-Wan trying to find a foster home for baby Luke, so he might reappear as well (albeit as a toddler).
That's all the ideas I could remember off the top of my head. I'll probably remember some more later. Anyway, thoughts and contributions welcome.