The thing is to keep it from becoming overly complicated. Old Jedi with some arcane agenda smacks of EU. I offered my idea only as an example of how existing dialogue could be used to allow for elements extending organically from the existing 6 films to exist. The antagonists have to have motivations as strong and central to their character as the heroes. So, if the Skywalkers are a family saga, make the antagonists also a family; if the heroes are rebuilding the Republic, give the antagonists an equally strong but opposite ideal to believe in; the heroes are users of the Light Side, you be sure have among the antagonists users of the Dark Side etc. That's how I would develop the germ of the story. So you can have mirroring scenes, parallels between the two stories. That's how I always write this stuff. Then you have opponents who can, to a certain degree, anticipate each other's moves, and it becomes a game of chess; but what's motivating the "game" isn't as as simple as a lust for power or a fanatical belief, it has to be organic, the way Luke's surrender was motivated by his compassion, and need to believe that he himself would not turn to the Dark Side, so he had to see that in his father. In the PT you came to understand Anakin's motives and you could feel empathy for him, if not sympathy; in the OT, Lucas continues that story. Now they have to create a similar strong line connecting the protagonists and antagonists. Instead of actually having them be related, I would simply have them mirror each other. Gradually, with each scene, driving the wedge in, until two possible allies become sworn enemies. Then you have two sides in irreconcilable conflict but your'e able to create audience empathy and emotion from both sides in the audience. Unlike Anakin & Obi, where the relationship was basically forced, have it be an unlikely alliance that works at first, then falls apart under strain. So at first you're cheering Luke on, like the heroes of Buddhist and Hindu epic, who convert demons and monsters, then you watch in horror as his enlightened intentions end up letting the serpent into the garden, so to speak.