Imagine if you will the world of Spira, made famous by its sole appearance as the world of Final Fantasy X. Two years has passed since the noble heroes Tidus, Yuna, and company have (shocking spoiler!) defeated Sin! Now, two years have passed, and a new threat is upon the land. What is it? That's soon to be revealed, but it isn't Sin. The technology of the Al Bhed is finally being used heavily. The temples of Yevon are now flying fortresses. The world of Spira has changed quite a bit.
The game, tenatively called Final Fantasy X-2, follows the continued adventures of Yuna the summoner, and obviously marks the first ever direct storyline sequel to a Final Fantasy game. Besides those small bits of story, the only things known at this point is that jumping and dashing have been added to exploration, giving the 3D world a bit more depth and a lot more reason to run around. Also new will be an Active Mode, which will employ Dragon's Lair-esque 'hit the right button at the right time' gameplay.
Speculation is currently that Square will formally announce everything about their first ever Final Fantasy storyline spinoff at their conference on the 22nd, this Tuesday, and will also have details in the upcoming V-Jump, a Japanese publication, which should also be hitting newsstands this Tuesday. RPGamer will have the details then, if not beforehand.
Source: Quiter / Games Are Fun