You know, I've been wondering the same thing.
There is a huge potential for making players activate all these open empty zones once more... perhaps as an on-going mission for the discovery (and disassembly) of "hidden" caches of Garlemald's "gifts".
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To be fair, we see this in a lot of places. The Hinterlands are as full of ruins and as abandoned, yet there's no actual reconstruction minus some tiny changes in Idyllshire. The Churning Mists are full of derelict ruins from the dragon-elezen heyday, and even though we've successfully put an end to the dragonsong war and they're learning to at least be neighbours again, nothing changes (other than the moogle area). It would be great to see improvements in all of these areas (including Ala Mhigo) but by this point I think that the XIV devs just like developing landscapes and don't want to put the effort into giving us reasons to be in it.
Indeed, and from what transpired in 4.4, I like the fact one of my favourite areas will gradually come back into reuse.
Ala Mhigo proper could definitely see a remake and the skyborne areas of the Churning Mists are literally begging for players to re-activate its spaces.
Some of it may be cut content. For example the rat people were going to be a beast tribe for crafters/gatherers but SE changed their mind and went with Namazu instead.
I'm hoping for something to use more of the Lochs in 4.5 but at this point I'm not expecting anything.
I would have LOVED to have the rat race get more prominence this time around --likely because I'm biased towards those sly Namazu, but more likely because I found the rat race thoroughly hillarious in FFXI. An example is Kakkaroon-- located in the ToAU area-- him and quite a few others made me laugh on several occasions.
And that their dialogue when it came to gil... "clinkity clink" and so on... priceless.
If the next expansion is going to take us into Ilsabard, then I think Ala Mhigo will be the main hub city for 5.x.