That aesir glider and the pristine, orderly look of the castle behind Baird is how I view New Argive: you have the past excavated and utilized for further academic pursuits, and you have bastions of learning (versus experimentation like New Tolarian Academy).
I think that's an excellent way to describe it.
We did just see more Lage in Modern Horizons with
Marit Lage's Slumber, so I wouldn't count her out just yet given that the folks actually making cards seem aware of her (unlike, say, the Carthalions). If the set needs a big conflict (as most seem to), Lage could be it.
Don't write off the Carthalions yet, there was flavor text in Dominaria.
Not only that, they actually wanted to include a card for Jenson Carthalion in
Dominaria but decided against it because there were already too many legends and planeswalkers whose name starts with J. Which has to be the silliest reason ever, but at least it makes me optimistic we'll see that guy the next time we visit Dominaria. Plus, there's
Fists of Flame in
Modern Horizons, and don't forget that Ethan is our secret weapon in R&D who is 100% aware of the Carthalions.
I think WotC's potential problem with Marit Lage wouldn't be a lack of awareness, it would be that Emrakul has usurped Lage's rightful place in the storyline and is now Magic's resident eldritch horror (I'm deliberately phrasing it that way because the Eldrazi sucked and their arc was a mess. They rectified part of the problem on Innistrad, and even then I didn't like how they eldrazified everything in
Eldritch Moon). They'd have to find new creative space for Marit Lage to play in, so to speak, something that would feel sufficiently different from the Eldrazi. It's a shame she wasn't the creative hook for
Coldsnap, really. Not that the
Coldsnap story was terrible, but it would have been more organic than making up an order of ice wizards we'd never heard of and introducing a battle that wasn't mentioned in the historical accounts given by Arkol in the
Ice Age trilogy... A flashback set about Jaeuhl's sidequest to stop Tevesh Szat's schemes would have been great, too, but I digress.
Beyond that, New Argive suffers from being placed adjacent to history with Sarpadia, which had a staggeringly longer lasting impact on the game's zeitgeist.
Mind you, I'm even fond of that trilogy with Jodah, but aside from that, my mind almost automatically slips to thinking of Sarpadia instead.
I get where you are coming from, the two are pretty closely linked in my brain as well. I think the most realistic scenario would be a set that has a strong focus on New Argive (in more or less the way in which
Dominaria mostly focused on the Domains) but that also has room for some other places. Most of the things that work for New Argive should also work for glimpses of Almaaz and Gulmany, and Sarpadia could factor into it as well. I think the most relevant parts of Sarpadia's legacy (and presumably the only creatures still living there) are the thrulls, thallids and saprolings (saprolings and thrulls were also mentioned in the recent survey). The set (or preferably several sets) could show us the thallids and thrulls that have spread across Sarpadia. I always imagined
Sporoloth Ancient might be from modern day Sarpadia since it's from
Future Sight and feels like an advanced version of the basic
Thallid.
Dominaria completely moved away from the spore counter technology, but reintroducing it for the thallids in Sarpadia would be a cool way to show how they've evolved in isolation. Maybe there were expeditions that went to Sarpadia and never returned, and their journals would make for great found footage style flavour text.
In a nutshell, I think Terisiare/New Argive wouldn't really take away too much from Sarpadia because the latter is presumably overrun by monsters and completely inhospitable, but there could be a thematic link between the two in the form of researchers who went to Sarpadia to study it.
Sidenote to modern day Sarpadia as a setting: As far as we know, some of the islands close to Sarpadia were (and perhaps still are) inhabited long after the Ice Age. The story "Foulmere" in the
Distant Planes anthology is set on one of those islands, and the Loot Niptil stories probably are as well. One of the redshirts on the original
Weatherlight crew is even called an Icatian, so their culture must have survived somewhere. It sure would be awesome if Loot Niptil's buddy Grover was still alive and showed up in a set, and the same area also has/had a place called Junktown (sorry, Automatown) that is/was full of archaeologists and artificers who were tinkering with old junk from ages past. But I guess most of that would stretch the focus of an actual set too much to become a reality, although I'd be super thrilled to be proven wrong...