First off, I would set it 1000 years after Return of the Jedi. Why would I do that?
1. Star Wars draws inspiration from epic fantasy and long term generational stories like Dune. It's not that odd to jump 1000 years ahead in a story like this.
2. Regardless of whether you care to keep the expanded universe intact or not, 1000 years makes this discussion less of a relevant point. If you never got into the EU or if you followed everything that came out, your characters lived long enough and enough time has passed for their victories to have meaning.
3. I can't help but feel like the Vader-Luke-Leia story, in broad strokes, did really end with ROTJ. I'm not saying I didn't enjoy EU stories that still featured Luke and Leia, but for your large, big screen, new Star Wars trilogy, the best way to avoid retread is to just move on completely.
I know there might be some that would say you could still get millage from the Luke-Leia Skywalker arc if you had one of them turn and created a new redemption arc, but honestly, I have to admit I'm not much of a fan of Dark Empire. It really felt to me that if you have Luke, the very next generation of Skywalkers, fall to the Dark Side, you have a really bad loop of Skywalkers being the best and then falling as soon as they hit the top of their game.
Either way, we if you liked Dark Empire, it could still have happened under this set up.
So you know I would shoot the franchise into the future--what would I do next? Well, the first movie played with a lot of themes that repeat themselves across real world history. Taking a cue from Rome, we've seen the "Republic becomes an Empire" on screen. What haven't we seen? "Rome is torn apart by outside invaders."
200 years ago (from our 1000 year leap forward) a Mandalorian horde not seen since the ancient days of the Old Republic formed. The Republic had about 700 years of peace and prosperity where it grew complacent and corrupt once again, but this time, there was no Sith Lord to manipulate the fall. The Mandalorians managed to tear apart the New Republic/Galactic Alliance, but the Jedi managed to kill the Mandalore, the horde fell apart, but with the galaxy still fractured.
Nobody brought anything back together. For a century sectors have warred against sectors, and a Dark Age has fallen across the galaxy. Hutts sell weapons to all sides and grow fatter. The shattered Mandalorian Clans sell themselves as mercenaries to various sectors for a price, but cannot regain their lost glory.
So what's going on with the Jedi? Well, they weren't wiped out this time, but they were scattered. There is no large central group of Jedi, and those that come in contact with one another are at odds over various changes made to how they teach and train their members. Some hold to the old ways, and train from infancy, never allowing their members to form attachments. Others recruit teenagers and form families. All of them believe the others to practice doctrines that, while not of the Dark Side itself, might be dangerous enough to cause them to fall, and they fail to pull themselves back together.
So who are the big bad guys?
It could be any Dark Side sect that isn't the Sith, but my preference would be the Sorcerers of Rhand. Why not Sith? Because at this point, even if we didn't realize it back in 1977, the Sith have been the ultimate bad guys for six Star Wars movies. They had their ultimate victory when Palpatine was made Emperor, and their ultimate defeat with his death. Again, if you were an EU fan or a movies only fan, this still bookends well.
So why the Sorcerers of Rhand? They believe in Entropy. They don't want to galaxy to be united under their rule. They want it to die and fall to pieces. They have a vested interest in the galaxy falling apart and withering. Why are they the villains now?
After a century of galactic dark ages and sectors warring with other sectors, warlords, monarchs, crime lords, and sector rulers are being called to Coruscant by the Tradionalist Sect of the Jedi. The Traditionalists want to begin to put the galaxy back together, but they are certain that they are the ones that should be anointing the new galactic government. This also brings some of the leading Jedi sects to Coruscant for the meeting as well.
The Sorcerers of Rhand have their own powerful warlord that they are secretly propping up for galactic rule. The warlord himself is certain that the sorcerers just want to be the power behind the throne when he takes over the galaxy, but in truth, the Sorcerers of Rhand want to use him as a hammer to smash the strongest alliances that have the greatest chance to bring the galaxy together, and then discard him.
The Sorcerers also want to kill the most level headed and forward thinking of all of the Jedi sects. After all, the Light Side promotes life and renewal. The Sorcerers of Rhand want the galaxy to finally fall into total darkness and annihilation. That said, they want their "candidate" to look like the hero, so they hire a mercenary group of Mandalorians to attack the conference. Their warlord drives off the bad guys (with help from some of the major characters we would be introducing), but one Jedi (our protagonist) finds out the Sorcerers are behind the whole thing. They manage to frame him for the assassination of a Traditionalist Jedi, and while the talks and all of the major galactic players are safe, our hero is on the run trying to prove the Sorcerers of Rhand are the real threat.
Our hero would be a Jedi that isn't from a major Jedi sect, but rather from a "Master to Apprentice" minor tradition in one of the smaller sectors. He doesn't really have much of a dog in this fight, because his masters have always just emphasized the baseline Jedi ethics we all knew from the first movies. And you you really need the cameo, our protagonist can be carrying a holocron created by Luke, so that we get to see Luke as the holocron's gatekeeper. We could throw in a Mandalorian merc that found out that they were being used as dupes, and a smuggler helping out the Jedi and the Mandalorian for the cash.
So that's my pitch. I'd have to cogitate a bit more to flesh out Episode VIII and IX, but I think it's a solid, workable start. Visually, the Sorcerer's patsy would be the cleanest, most visually uniform of all of the factions, since he's got the resources from the Sorcerers on his side, so he's got the most "Stormtrooper-eque" troops on his side. I'd co-opt the darksabers introduced in the Clone Wars animated series as the weapons of the Sorcerers of Rhand so that we would still have some lightersaber on lightsaber goodness, but we keep the color coded evil convention from the previous movies.
I'd have the Corellian Sector as one of the most powerful factions, and one that doesn't want to run the galaxy themselves. They are almost the "light side" version of the Hutts, as they prosper from selling ships and the like to multiple sectors, but with less of an emphasis on keeping everyone fighting one another. The Corellian Sector representative really just wants to make sure the new Republic or whatever has a strong foundation, without directly controlling it. This ties into the EU, but also shifts the known political movers and shakers from Naboo and Alderaan (obviously) to a new player, but one that has been reference both in the EU and the movies.
Thoughts, jeers (okay, not really), and ideas are more than welcome. It's all just really fleshing out my conjecture for fun, because of all of the few bits and pieces we know about the actual movies, we know it's nowhere near what I just pitched.