Possible spoiler warning!
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So there is a theory floating around (not new) about what will transpire in 5.0. But I'd like to put another theory (again not new) into context with the trailer. Please bear with me.
The trailer opens with what we might assume is the Warrior of Light.
But imagine that it is actually the Warrior of Darkness, wandering around through a barren First world. (His home world) A world that appears to be bathed in light; streaming from the clouds.
Suddenly he is flooded with the memories of the Warrior of Light's exploits/journey. Flashes of the WoL's memories stream through his mind, as he holds his head. And as his hand lowers, he is now a vessel for the Warrior of Light. He is in essence now the player's character on the First world.
He's instantly greeted by a creature that tells him there is "no fight left to fight." Which would be a taunt to the WoL that he has made a mistake in coming to the First world. He is called a "fool" and told there is "no life left to live." The WoL basically arrived right at "the end... his end."
However, the WoL summons their strength. They fight back. Determined to become the First world's champion. The Warrior of Darkness.
Meanwhile, the WoL's friends are left to deal with other problems. Perhaps in the WoL's absence, which could be what Y'shtola is meaning when she says, "Until our friend returns, I will hold the line."
Thancred is protecting a most likely "reborn" Minfilia. Perhaps Minfilia is on the First world as well, in a new vessel, attempting to help reclaim the world as promised. Or perhaps she has come back to our world to help out. Either way, she is obviously not just a simple past version of Minfilia.
A lot of people are thinking alternate histories, or alternate futures, and the whole light vs dark theory. So I just wanted to toss this one out there in light of the extended trailer. It's just a theory though. Let me know what you all think.
Edit: Looking at the trailer again, I noticed after the SAM and Zenos clash, the WoL has an odd look on his face. Almost as if he lost the fight. Then it quickly goes back to the other scene in the strange new area. Maybe we lose on purpose (by design) to allow ourselves to be transported to the WoD's world.
Also, the trailer does end with "become the Warrior of Darkness." Maybe that is quite literal?