Personally, I'm of the mind that it won't be a look into time travel, per se, and we'll explore The First in order to bring balance and stability to both of our worlds. Maybe time passes differently in other worlds, and the Warriors of Darkness and Minfilia will have been there for a while waiting on us. What if the child in the trailer IS Minfilia, born into that world to help save it?

It's been such a short time since the last Calamity in our world, so maybe the reason all these souls are being called now isn't to prevent a calamity that begins in our world, but one that will affect both us and The First if it comes to pass. Maybe we'll be able to travel between both with the Scions on one plane of existence and the armies on the other and in accomplishing our goal, will bring down the misguided in charge of Garlemald.