Speculation:
Given the current resources available left in ToA, S-E has very left to work with in terms of a new 'end-game region'. If we are to speak of equivalents, Zilart had Tu'Lia and Promathia had Lumoria.
There aren't really any extra areas left for them to make an entirely new region, but they might be able to play around with what they've got.
I highly doubt that the 4 remnant areas were originally meant to be for Salvage but they'd converted it into that.. And since Alzadaal is still shrouded in much mystery in the mission story, theres a good chance a later part of the story will unlock access to a deeper part of Aldazaal.. and from the fact that it's the only region left in the ToA expansion where there are zones yet to be used or are underused, theres also a good chance Alzadaal will become the sky/sea of ToA.
My hypothesis.
Nyzul Isle was originally meant to be a staging point for assault but it's not been used, so instead of using it for future assault, they're going to convert it into a new zone. (they did the exact 'opposite' of this for Limbus. They converted lower promy and ou'phat obelisk for it)
If you look here
http://www.ffxi-atlas.com/maps/nyzul-isle/general/at the Nyzul Isle map, there are lotsa places where they could stuff in new mobs that drop trigger items and the like, and places where theres room to put a ??? or whatever it is needed to pop the NMs. (Its not particularly expansive but would look pretty good imo... the last map would make for a nice, albeit small place for a showdown with a strong NM :D imo)
But it wouldn't seem right to bunch everything into 1 zone alone, so they could get trigger items to drop in Nyzul, and then place ???s in Aldazaal as well (esp in those weird empty rooms near the Remnants) so that people can play with NMs in there - in this way introducing a tier system.
(eg. various triggers in nyzul > 4 popped tier 1 in aldazaal undersea > 2 popped tier 2 in nyzul > super nm in nyzul last map

)
In addition to this, theres a very convenient staging point that lets people gain access to the region and if this is once again a repeat of trends, both Sky/Sea are now extremely accessible zones once you meet the requirements - especially if you consider that Teleports are the most basic system of getting around anywhere in Vana'diel and staging points are the system in place for the Near East.
I don't know why im so interested in this happening, i just want them to make the popped NM structure for ToA happen soon.. as well as new monster types and the finale for ToA story. they need more mission BCs..