Whether it's one or more that turn to the Dark Side I really can't say, but supposedly that's going to be the core of the story.
...which already pisses me off a little bit. Two great and decent persons like Leia and Han, and their kids turn into some of the biggest Sith threats since Palpatine? I actually hope they'll ignore or change that, even if it pisses off millions of readers.
IMHO they almost certainly will wipe the slate clean or close. It would be a nightmare to try to use the EU material and (main) characters. There are decades's worth of back story (and I think the EU timeline has now reached the years that movies in which Ford/Hamill/Fisher at their current ages would cover). The only way they could turn the new trilogy into a continuation of the EU is by making those movies for the EU readers and forget about the rest of us.... which of course they won't do, as for all the readers of the EU, the movies have much bigger audiences.
A major problem would be that the EU's "new generation" of Solos/Skywalkers has WAY too much back story and baggage already.
The EU has also milked the prequels's themes way too much. To moviegoers, another story of downfall to the dark side or having fallen to the dark side, which in movie form would be very simple, would be a total rehash.
My guess is that Lucas probably came up with a new interesting villain/problem, something that isn't a rehash of the existing saga nor something from the EU.
At the time of the PT, the notion they would disregard the back story stuff in the Zhan books caused uproars and the drama queens spoke of the "end of the EU". Then it all died down and the EU kept selling.
