For those hoping for a world revamp, plus new zones, plus a new class, plus new races, plus more customization, plus housing... I predict... disappointment.
I think a combination of "we heard you" and "we totally don't have the manpower to do much else" will result in a fairly basic expansion without big overarching systems.
They might keep some form of Torghast, with very similar powers and mechanics, just with a different theme (maybe a return to Island Expeditions, in name only), but mostly or entirely for cosmetics.
Similarly, there may be cosmetic covenants with the flights, but sharing stuff like the mission table and any borrowed power they may have, if any.
Races seem more likely than a new class, if only because that skips having to deal with old content. Maybe Allied Races, which require less work and can be introduced in bulk. Class skins would be nice, and relatively low effort, but I don't think they see them as a popular enough feature at the moment.
As for story, probably something a bit more light-hearted, going for a "return to adventure" feeling (that's totally not parroting FF14), about joining forces, healing the world and all that. Xal'atath, Azshara, Galakrond, Chromatus... they all sound as decent single-tier villains for a rather unfocused expansion.
The wonders of technology and an absolute lack of artistic skill.