So err... The hooded guy doesn't have a tail, nor apparent cat ears, as one would expect from G'raha Tia. Then again, nor does Y'shtola in the trailer... Could just be obscured by the robe I guess, not like these trailers usually make glaring mistakes... *cough*Nidhogg*cough*
Naturally, I want it to be Lahabrea, filling a Unukalhai style role in the First... More so since Gaius muttered something about missing his chance for revenge against Lahabrea...
I could actually see it being Unukalhai, come to think of it... Just because he is maybe going to need a new room to stand in if Minfilia is returning... From the features we do see of the hooded figure, he does seem fairly young? That's also my assumption though, just since I figured they'd want a shota Ascian to mirror the new look Minfilia is rocking... Unukalhai also has a track record of surviving world ending calamities, only to move to a different world after... Heck, he was the first Warrior of Light to flip sides too, and the only surviving one... Makes sense for him to mentor us in that process...
EDIT: Oh, also Unukalhai is currently chilling in the Rising Stones, which is right next door to Crystal Tower, so he is in a prime location to teleport the whole thing if needs be. G'raha is even closer of course, but also apparently in stasis. Given I believe it was stated we'd need to open the Tower to end that stasis, I imagine a "Worst Future" scenario wouldn't result in the Tower being opened, meaning he is probably powerless to really do much... Unukalhai taking a page from Galufs playbook and teleporting the it away, mixing in some Elidibus tricks to take it to the First as well? Then Crystarium is built up on the First specifically to open up the Tower and get access to G'raha and his Royal Blood... All in order to do... Whatever it is...
If nothing else, it's a nice explanation to why Crystarium isn't a mess like Bizarro-Limsa. For a world on the brink, it looks oddly like a utopia... Although I guess if you had access to Crystal Tower, that'd probably just be a given, heck it was basically a glorified solar panel, and they live in a world of perpetual light... It should be producing twice as much power as it was at the Allags peak...