My question to you is "what remaining territories?" We've effectively taken everything that isn't Ilsabard proper by this point in the story if we count side quests. Corvos is like a border state so I count it as part of Ilsabard, and iirc Thavnair is considered Southern Ilsabard, aka the none shit side unlike Northern Ilsabard
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So using this map as reference (Edit: KF hates FFXIV's weeb geography for some reason), the issue to a full Garlemald expansion (zone construction wise that is) is all that white cloud stuff in the Ilsabard area is effectively meant to be a brutal snow and ice wasteland, which makes for a very repetitive zone design as Garlemald's entire zone theme is a bitter land where only the strongest could survive as a bunch of poor cursed fucks who had to crawl into some semblance of power and stability by making their own merits without cheating with magic. The coldness further implies their sheer grit due to being able to survive in such a climate up to this point, which can explain why Garleans are tough ass bastards who can survive without full blown magic. Plus we already got Coerthas for the snow wasteland zone.
Bozja is iirc either just above or below Dalmasca and the Othard is already liberated so the eastern foothold (and half the reason the Popularis wanted to make peace with Doma is because Doma is critically close to Garlemald) is cleared, Ala Mhigo is liberated so that western front is already done. Corvos is near Thavnair which maybe you could push for about 2 zones with those areas + Garlemald proper, but you'd need some really weird excuse for why we can't just go through Othard or Ala Mhigo to justify going through Corvos because air ships are a thing.
This also presents the eventual question of how we introduce the Sharlayan isle later? This isn't the same as say Othard/"The East" which still has the entire Higashi isle to explore or even going through the great scar itself if you want to go that far. This isn't even touching Dalmasca or Bozja proper. Ala Mhigo is a bit harder to flesh into six zones, but you could probably have put Werlyt or some kind of border base that hints what the harsh difference between Aldernard and Northern Ilsabard to then hint at Garlemald later.
The six zones requirement quota I feel cucks a lot of potential ideas for expansions because you know, and I know, that everyone (including some of us probably) would at least somewhat complain about them skimping out on zone content if they said Garlemald expansion only had say 3-4 zones instead of 6, even if it does make sense for the story. People to this day still whine and complain that we don't get more dungeons, when dungeons fucking suck and do nothing, at least zones are fine locales to explore as back drops for the plot while dungeons suck literally after you do it for the first time in 20 minutes.