The story is different for each Final Fantasy game. There are two sequels to some of the numbered games if I recall correctly, which is FFX-2 and FFIV: The After Years.
We barely know anything about the story of FFXIII though. All I know is that there's something going on with the Fal'Cie and the L'Cie (don't know if that's how they're called). They're two groups of people or something, and one is good for humanity and the other isn't.
The battle system... well, once again it's pretty different all the time. Too bad you never played Xenogears, but it looks like it a bit. You have a bar that has for example 5 action points. Each moves you want to use cost action points, and you can create combos with different moves as long as you have enough action points. Some attacks that cost 3 actions points are more powerful than ones with 1 action points, but you do less combos. I guess we'll see how it works!
Random game thought :
Why is Bionic Commando Rearmed 2 getting so much hate? We finally get a real game and they're not even satisfied... I'm starting to hate the gaming community so f****** much...
