To be a bit more direct and less tongue-in-cheek: Gelmorra's a weird one among plot threads, in that we've had sort of the 'beginning teasers' but never any sort of payoff to it.
In 1.0, Gelmorra was clearly being set up as a major element; they had three dungeons (Tam-Tara and Toto-Rak, the Mun-Tuy Cellars were a dungeon then too), there were a bunch of ruins scattered around the Shroud, the endgame currencies were from there, both the history of the Shroud and the history of an entire race is rooted in it. Chances are, they wanted to make it the same sort of story presence that Allag ended up being in the live game, this constant shadowed presence of those who came before. I do think Allag was better suited for that role as an aside, because the intended endgame of 1.0 would've been heavily skewed towards the Shroud, which would kinda suck for anyone who started anywhere else.
But it leaves Gelmorra in a weird spot in the live game, because unlike a lot of the 1.0 setup they threw away like the mystery of Silvertear, Gelmorra couldn't be completely brushed away; there was just too much of it. So as a result we're sort of left with a conspicuous amount of setup for a setting element that has no payoff. A lot of setup for 'this is a place that was real, these are the people that lived there, these are the still-relevant societal issues that stem from this time and place', and then... nothing. Ever. I did a whole video on PotD's lore, and the fact it was Gelmorran (well, a quarter of it was) is basically incidental to the story present about the whole place.
Sure, if there's a time-travel expansion, visiting Gelmorra in its prime would be neat (although I'd expect a time travel expansion to go to the Third or Fifth Astral Eras, not early Sixth), but if that's the only thing you think can be done with it... well, I can tell you're not a desperate and frustrated Duskwight roleplayer, at the very least. There's indication there might still be a Gelmorran population somewhere down there, for one, which certainly has potential. On top of that... while we've been to Gelmorran structures, it's never been because of Gelmorra. We know next to nothing about their actual society, art, techniques, history (beyond 'was there, kinda sucked, then people left', which sounds to me like history written by a Gridanian who wants to marginalize that part of history), anything. And I dunno about you, but 'underground society that thrived despite all above-ground nature literally wanting to murder them' sounds like a really neat setting, especially when you mix in the kinda suspicious history about the Padjali and the not-total migration above ground, which seems to me like something fishy happened.
For all the times we've been there, we've never had an actual Gelmorran story. We've overdue for one!
EDIT: Iscah gets it! It'd be a beautiful setting if they actually took a swing at it outside of the ARR constraints.