I do not see how they cannot draw some of the story line of the new sequels from books already published. I would hope that the three sequels conclude before the Yuuzhan Vong War, too dark a period for younger fans and Disney imho. So far as Sith or Dark Side Jedi, they always makes things interesting.
Well they said that they were not going to with their statement that it was going to be a whole new storyline.
So George Lucas lied yet again.... He had stated that Episodes I - VI were his story. No more, no less. I have never seen a creator retcon himself as much as George Lucas. He's lied and backpedalled so often, I don't know what any of us can believe out of his mouth, anymore.
George did not lie. There were meant to be 12 movies. He then changed his mind to 9 and then 6 because he was tired of Star Wars, but the outlines were still there and those have been passed to Ardnt. You can read all the different ideas and changes here
When did I say there would be no dark side?
My point was, there needs to be an enemy using the dark side of the force, and likely a red lightsaber. However, the dark side user(s) should be starting from scratch, just like Luke is doing with the light side. His "Jedi" are much different from the Jedi order of old.
The Sith line died out with Sidious and Vader. There will be baddies who will come up and fill the power vacuum, but they won't be genuine Sith.
Going by that philosophy, the Sith died out at the end of the Great Hyperspace War. They are not resurrected again for 600 years until the spirit of Naga Sadow converts Freedon Nadd. So this is a NEW Sith, not the orginal. It's Freedon Nadd's take on the Sith. They die out again and are once more resurrected 400 years later by Satal and Aleema Keto. Then Exar Kun takes over their Krath Cult that is based on Sith teachings. It isn't until the spirit of Marka Rangos crowns Exar Kun Lord of the Sith that they are thrust onto the galactic stage once more. And so on and so on. Leaping through time each time a different version of the Sith. So the REAL Sith, as a philosophy, died 4000 years prior to the one we see as the Sith. The one formed by Darth Bane. This being the rule of two, only one master and one apprentice.
So to your final line "There will be baddies who will come up and fill the power vacuum, but they won't be genuine Sith." Sidious and Vader weren't really "genuine Sith" either.