That roar at the end got me surprised, and then I had a thought, what if it's Azdaja?

Vritra clearly seems to live in Thavnair, he appears there i the trailer and Vritra being a mythological creature in hunduism, it would match with the vibe that Thavnair gives.
So then where would Azdaja be? Clearly not in Othard. He could be in Thavnair with Vritra but Ilsabard could be a good option.
Azdaja is also reference as a mythical slavic, so him being in Ilsabard could make sense (Garlen empire giving heavy imerial europe vibes with particulars to Russia due to huge size and almost always depicted as snowy).

Fandaniel also mentions that the goal was to siphon the Aether from cartenau to Garlemad, with that not working, they have to get it from the dreamer. A creature with huge amounts of Aether screams dragon to me.

Finally, with Endwalker being the end of Hydaelin and Zodiark arc, I would expect to know the situation of all the first brood too as they have a close link to Hydaelin.

So yeah, my theory would be that Fandaniel has Azdaja trapped under the Garlemald tower, using it's Aether to create a primal to temper everything (possibly Anima based on other theories) and they have to use more of it's Aether for that Gateway to the gods thing instead of using the Aether from Cartenau.