There may not be a new major arc as large as Hydaelyn and Zodiark. But if there is, one question is who will the new master villain be? Someone we've encountered before? Someone new?
But what if there isn't a villain at all?
Etheirys was split into the source and thirteen reflections, which existed in some kind of equilibrium of mass and energy, however that is defined in terms of the physics of this fictional universe. But then the Ascians started their rejoining project, unbalancing the aetheric energies of one reflection after another and causing them to be reabsorbed into the Source resulting in a calamity on the Source.
That confirms the idea of an original equilibrium between the fourteen worlds that had been formed from the original, and that they affect one another. Simple and obvious conclusion, right? Especially when travel between reflections and the Source are possible via multiple means.
So, what if the Ascian meddling, which led to the destruction and resorption of several reflections, had led to an imbalance in the metaphysical/energetic alignments or orbits or however we picture it? Like knocking atoms of out a molecule or planets/moons out of orbit? Or simple reducing the necessary interplay and levels of substance and energy of the system?
If some kind of imbalance or structural disruption to the fabric of the sundered star has been introduced (which would have been irrelevant to the Ascians as they were planning on a full forcible rejoining, what might happen to the Source and remaining reflections? One possibility is, to borrow and modify a line from the Endwalker trailer, "the fabric of star has begun to fray...", and then, to unravel? I'm not suggesting this was a hint or clue left for us, but it fits my suspicion of the big multi-expansion threat perfectly.
It is, of course, conjecture, but it would work on many levels. For example, the "former" Scions are studying a more reliable way to travel between the remaining shards of Etheirys, so they would be studying their substance and connections. How they fit together and influence one another. Perhaps they also have to find the right resonance or frequency of each shard and then try to map things, seeing how they interact with each other. That could lead to further analysis and the realization that the shards are starting to slowly fall apart or unravel. Maybe this doesn't take place right away, but it becomes a plot point.
Now we have more urgency to visit other shards. Not just for exploration or adventure, but to take readings and measurements, to set up some way to slow or counter this imbalance that is gaining speed and threatening all shards. But of course, we would have to solve some set of crises on each shard to get the cooperation of the people living there, providing a unique set of adventures. Perhaps in the end, we may even have an anime moment reminiscent of the trailer for A Real Reborn where the peoples of all shards come together in their intense hope (another use for the concept of dynamis?) to reshape or reform or otherwise stabilize their individuals worlds within the greater whole? To create a new balance?
Anyway, there is no reason any of this should be true. It's just fun speculation. Thanks for reading.