The first narrator is Zenos, as the Warrior of Light remembers; dialogue and then a battle.
It's shown that it's a memory when the camera pans out and the Warrior of Light is staggering through wastes, being chided by an imp.
Thancred is shown escorting a shorter being through a building when they're attacked.

I think once the other two Fanfests complete, we will have a FULL trailer. At the moment, I think with what we are given, it's fairly straight forward. Balance between light and dark is tipped to much and we're on the precipice of another Astral era. Main plot will be fighting Garlemald, over-arching plot will be finishing the Ascians. Thancred has swapped over to be more offensive and is touting a gunblade, while the Warrior of Light is going to have to work for the darkness for a bit! I don't think that it was Arbert, I think that's us!

I'm hopeful that they wouldn't introduce time-skips, temporal anomalies, or any other timeline changing to the story.