If the main character is force sensitive, all bets of a new story are off. And as long as you stay entwine with the Empire, the Death Star, Siths, Storm Troopers, etc, "new" isn't going to accurate.
It's one thing to tell the story of a young force sensitive figure joining the Rebel Alliance and facing an evil Empire and it's giant spherical planet destroying weapon.
It's another to tell that story with a youth on a desert planet with unknown yet important parents who finds a droid with vital information, an escape from said planet in the Millennium Falcon, a trip to a shady cantina, a wise Small sized mentor that has been around for centuries who instructs in the Force, a shield generator that needs to be deactivated, an X-wing performing a trench run before blowing up a spherical planet destroying superweapon, a fallen Jedi in black with a mask killing an aged mentor, and the like.
You can play with a lot of the iconic Star Wars elements and even touch on familiar plots and tell a new story. They did it with the prequels, they did it with Clone Wars and the follow-up series, and they did it with many of the comics and novels. Heck, they even did a plot with a fallen Jedi, evil Empire, superweapon, and more similarities in Knights of the Old Republic and it worked without being a carbon copy and doing its own thing.
A new story is very possible.
Rogue One is Disney's chance to show they're willing to do more with Star Wars than just milk nostalgia. Or it could just be a mishmash of homages, Easter Eggs, and rip-offs.