In my opinion, the opening trailer was an echo to a real event in the past.
The Echo is a mysterious power which causes the souls of those touched by it to resonate with others. Of that power is born the ability to relive the other person's past experiences -- to see and feel them as though they were one's own.
I think in this case the levequest tile that the midlander picks up was a duty to kill a marlboro that was previously used/completed by a group 10-15 years ago with one of the 4 party members having the ability to Echo. When the midlander, also with the Echo ability picks it up, his soul links to the soul of the 4th party member and he is mentally warped back to the time of the 1st party and is reliving the events inside his body (we see our midlander's face, but the party members see who they think he is). This explains why he changes from a Gladiator to an Archer.

The battle progresses and then comes the part when he is scooped up and gets flown to the battle of the Garleans vs Midgardsomr. When the ship crashes into the lake in Mor Dhona, it releases/explodes a vein of aether which disrupts the aetheric veil and allows the Primals access to this world (you see Ifrit and Titan being released amongst others). Then the 4th party member falls (and I presume he dies) still with the tile on him, which is later recovered from his corpse and repurposed in the Adventurer's guild, waiting Echo-sensitive players to touch it and relive his last days.

In the new trailer he is a Warrior (cause lets face it, for a long time Marauder/Warrior was a better tank) and the Miqote from his current party is there as a bard. The Elezen lancer could possibly be the blackmage. However while the green skinned roegadyn is similar, as with the lalafel whm, the female elezen gladiator isn't there, so I don't think it is the same party, just similar looking people. I mean 10-15 years is a fair amount of time. The adventurers from the echo may have retired.

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However, I have debated this with my lsmates a bit as in our main scenario Echos we have not changed class or become someone else. Also we had discussions on wether or not echos were real in the sense that you conversing with someone in a echo to the past would change the circumstances in the future, but most of our talks were inconclusive.