A lot of the story going forward is dependent on how 6.0 ends. Whether there's a timeskip, a shunting of WoL off into another shard, it's unknown. But we are supposedly leaving most of the old cast behind after Endwalker, so that should be kept in mind. However, as far as theorization goes, I think people are far off base when saying that the threat of the primals will be over in 7.0. Just the opposite, I'm fairly sure that they're set up to be a much worse threat after Endwalker.

As to why? Let's just look at what's been set up and what little we know about the story post-6.0. In the EW prepatches we discover the cure to tempering and bring the various Beast Tribes into the Eorzean alliance - On the face of it this sounds like an attempt to put the issue of the primals to bed, but I would put forward that this has been done because the issue of the primals will be badly exacerbated in the future, and so it's no longer possible to just have the Beast Tribes summoning random primals across Eorzea based on solvable grudges with the city-states.
Just consider the two side questlines we know of after 6.0, Pandemonium and Myths of the Realm. If Pandemonium is similar at all to FF2 which it's referencing, it should presumably have to do with some realm of "hell". This is of course quite strange in the context of XIV's Lifestream-based reincarnation cosmology. And then we have Myths of the Realm, dealing with the Twelve and, notably, whatever lies underneath Silvertear Lake.

For what this means, we need to think about the nature of primals, their origins, and what we actually saw during the 1.0 cinematic. As we well know, primals are constructs born from faith and prayer, only one can exist of a given diety at a time, they must feed upon aether to sustain a physical form, and when they're defeated their essence returns to the aetherial sea to reside there until it's summoned again. However with what we now know about the nature of summoning magic - That it is in fact a primitive version of Amaurotine creation magic - Their entire existence becomes quite confusing and questionable. For example, why can only one incarnation of a primal exist at a given time? Creation magic allowed the Amaurotines to create as many copies of a concept as they wished. Why do the essences of the primals return to the Lifestream upon their physical dissolution? No such thing was the case with Ancient concepts. And why must primals feed upon aether in order to sustain themselves? Again no such thing was true in the time before the sundering, while Zodiark needed huge sacrifices in order to preform his reality-altering acts we hear nothing about more sacrifices being necessary to maintain his existence, and Hydaelyn has been weakening for 12,000 years but seemingly not just because of her nature as a primal. Moreover, the inclusion that modern deities seem to be based on specific Ancient concepts, such as Ifrita to Ifrit, or Amaurot's Phoenix to Louisoix' Phoenix, further complicates the issue.

And then, the question of the 1.0 cinematic. There, we see what appears to be the essences of the primals being freed from Silvertear by the massive Ceruleum explosion of the Agrius. How and why could this be the case? Well, it seems to me that the concepts of the primals were effectively being locked away there by Hydaelyn, and when the seal was cracked they were released, to be freely drawn upon for summoning once again. As an aside to this, Lahabrea's entire thing in the Ancient world was experimental concepts, and his plans throughout 1.0 and 2.0 hinged on the summoning of primals, so it seems likely he was quite involved with all of this. It also seems his primary goal was the enervation of Hydaelyn, which brings me to a few conclusions and postulations about all of this.

1. What lay beneath Silvertear was effectively a prison for Amaurotine concepts, to prevent their modern usage for summoning.
2. Hydaelyn was responsible for being the force that draws the essence of the primals back into the Lifestream, in which resides that prison, after the destruction of their physical forms. I would further speculate that this is why the primals need a constant influx of aether to maintain themselves, because the pull of the Lifestream is so strong that they need a huge amount of energy to sustain themselves.
3. Lahabrea's intense Primal-summoning routine was intended to drain Hydaelyn of aether, through both the summonings draining the aether of the land and through their defeats forcing them to be drawn away again.

For the moment, let us assume this is the case to at least some degree. At the very least, we can see that Hydaelyn is in part responsible for trying to tamp down on the possibility of summonings. And I would suggest that this is an ongoing and constantly draining process for her, which would strongly explain the massive explosion of summonings, and hugely increased ease of summonings, after Hydaelyn's expenditure of her power during the finale of 2.0. Compared to a few decades ago we went from primal summonings being unheard of to individual people being capable of it if they're supplied with aether. Therefore, what would happen if the outcome of Endwalker is as it seems, and for one reason or another Hydaelyn is destroyed?

If it is the case that she is the one pulling primals back to the Lifestream and forcing their constant consumption of aether merely to exist, would it not therefore be the case that in a post-Hydaelyn world, primals would be able to exist indefinitely, growing only more powerful as they sup upon faith and aether, and the only way to destroy them would be to shatter their essence or otherwise contain it somehow? If this should happen, then the threat that primals pose would be dramatically increased - Which would furthermore explain why the very first threats we need to deal with after 6.0 is that of the Twelve and Lahabrea, the deities with the most widespread reverence and the man most invested in the creation of concepts and summoning of primals. This also explains why a cure for tempering had to come about in the story now, because after Endwalker it will be completely necessary narratively. Moreso, such a situation would be much more conducive to a series of smaller-scope expansions, since primals would be able to serve the role of primary antagonists rather than side hustle.

Now, with all that preamble out of the way, I suppose I'll offer a theory for an expansion following Endwalker.

As others have suggested, I would be interested in seeing another Far East/Hingashi expansion in the near future. Hingashi is an extremely isolationist nation, about as much as Ishgard was, so they likely have some hidden reason for being so. They're also wildly corrupt, so that's another reason to swing over. And furthermore while we dealt with Susanoo and Tsukuyomi in Stormblood, we never encountered the apex of that trifecta Amaterasu. We've also yet to see Nagxia or Dalmasca proper, and with the Imperial storyline seemingly coming to a close the question of what comes after for such provinces could be quite interesting, as Matsuno noted that was really the story he wanted to tell after Bozja rather than another liberation narrative. With a soft reset caused by the disillusion of the Empire, defeat of the 4th, and oncoming apocalypse in Endwalker, it could be none of the previous content in the area would even be made required by the MSQ. Aside from all that, an expansion in Meracydia also has potential - If anyone is left there who worshipped the Warring Triad or Bahamut, well, we killed the incarnations of all four, so they could be resummoned at any point and pose an issue.

Like others have said, I'm not up for a time travelling expansion, but I would be interested in a timeskip expansion, jumping forward quite a bit. Alternatively it would be fairly simple to frame a trip into the past as nothing less than an extended Echo vision, perhaps living through one of WoL's past lives. This would certainly be preferable to physically going back in time yourself.

Quote Originally Posted by Cleretic View Post
Remember that there's a stated reason the Source doesn't have any known Ancient buildings (at least, that we're aware of); it's gone through a huge amount of apocalyptic, world-rending disasters, which eventually tear down even the pretty sturdy Ancient architecture. Consider Amaurot itself, off the coast of Vylbrand; in the First it had to take the End of Days, the Sundering (if that did any damage) and then eons underwater, while in the Source it's had to deal with all that, plus seven apocalypse-level events.

That's not to say we can't go to more Ancient buildings (even if I don't personally want to), but rather that any reasoning for getting us there has to be a little smarter than 'we just find them somewhere'.
Given the ruins on the First were all under the sea, it could be they simply still exist on the Source and nobody has found them. As far as I can recall we haven't exactly had the relations to ask the Sahagin about it.