I'd be surprised if the new movie isn't an original story despite how popular the Zahn books are to fans. Focusing on a new generation of Skywalkers seems like the natural progression if they're calling it Episode VII. Even if they don't follow any specific story from the expanded universe, I hope nothing gets retconned though I believe Lucas is on record as saying the movies take precendence and the Expanded Universe is separate so maybe there are no issues with rofl-stomping the Expanded Universe. He's not in charge anymore so perhaps that has changed.
As far as the movies go, collectively the original and prequel trilogies tell a complete story with a fitting conclusion. Return of the Jedi doesn't lead into an Episode VII as naturally as Revenge of the Sith leads into A New Hope. Additional stories can always be told and with the Expanded Universe, they have. As a movie franchise, I'd rather see Disney and Lucasfilm "start over" like Knights of the Old Republic did so they don't have to adhere so closely to the movies that Lucas made.
That's great and all, but who's going to play Luke, Leia, Han etc.? Certainly not the original actors, and recasting new actors in those roles would be equally insane IMO.
Tron: Legacy had a CG young Jeff Bridges. Like I said above, I'd rather see a new movie set in a completely different era as Lucas' films. If they insist on picking up after Return of the Jedi with Luke, Leia, and Han, I'd imagine CG is the way they'd go with it. Not saying I advocate it, but that's the likeliest scenario rather than recasting.