if i may put my two cents
from the trailer it seems like we won a decisive battle with Zenos and the Garleans. Which in turn could start a huge revolution and freedom across the realm. But also in the trailer we see the WoL in an empty and barren landscape.
If I may speculate, we win the battle as I said but then Hydalyn decimates the battlefield killing everyone expect us. Pretty standard. The fact that we know what happened in the battlefield makes us question everything we did. Of course a fight with light (whether internal or external takes place.) Dark Knight makes more sense since in that Storyline (SPOILERS) you actually fight your inner darkness.
I believe the struggle is internal and we finally accept our inner darkness to combat the seemingly nondiscriminatory light which killed everyone on the battlefield.
Hopefully the story takes us on a journey back to Eorzea as the WoD and our character feels like they had forsaken the realm and must commit a dark act to save it from the "Flood of Light". In general we are rewritting history as this same fate happened on the original WoD shard and our new allies might be the WoD we fought in 3.x.
I believe that would honestly make for a better story and would fit in with the themes of Final Fantasy