As we can see in the trailer, Thancred? will get his second eye back, but he don't use dual wield style in combat anymore. So, from this, we can put few Theory :
1) We well go back in the past because the balance between Light and Darkness is crumbling. We are constantly growing stronger and stronger. So for preventing the world to be consumed in the Light, someone summoned us back in the past to become the Warrior of Darkness and retablish the balance. But, by doing so, we'll change the futur. Like a Futur where Thancred wasn't send in the Aether and was able to keep his two eyes and even got a gunblader combat style.
2) We are send in an alternative world, where Thancred is clearly different from the one we know, even to the very basic of the character. And in this World, we see a world, who's about to be swallowed by the Light. And for saving it, we must become Warrior of Darkness.
That's my opinion of the plot.