I had some crazy theories...

1) The main threat from EW is what caused the Final Days, a creature (alien originated?) that is currently sealed in the Moon. That's a reference of how Bahamut was sealed in an artificial moon, as it was something taught by the Ascians to the Allagans, because they had an experience with sealing a threat before in a similar way.

2) The main tower structure in Garlemald will function like the Lunatic Pandora does in FF8 - a beacon to cause something like the Lunar Cry, where both this threat and a number of creatures will rain into the Source. Even the designs and the sheer number of monsters in Estinien's section of the Trailer, they all seem terribly similar to the creatures originated from the Lunar Cry in FF8.

3) Alternatively, it would be too crazy to think that this threat will take heavy inspiration on Chrono Trigger's Lavos?... The whole End Days narrative, voice from below, and even the main Garlemald Tower spiky design itself, reminds me a lot of Lavos.