Well there are bits of 2.X, 3.X and most of 4.X that had nothing to progress the Ascian storyline. We have been dealing with most of their attemps but the real story about the Ascians really started in 4.5. I'm also having a hard time believing 5.X will be the final end, but rather I see that story trickling into another. We know more about Ascians, what they were and what they do, but it brought in a whole new plot around Hydealin / Zodiark, about creation Magic, about their doomsday etc.
The main unresolved point is what will be going on in Garlemald, which I guess would be a fitting scenery for the 5.4, 5.5 and 6.X cycle.
What comes after, well the game will be around 10 years old after that (even more if we count 1.0). I'm not sure they are ready to plunge into another 10 years long storyline. I can see them doing micro stories. After all, there's still Old Sharlayan, Meracydia, the New World, Thavnair... All these places could have a short focused story to keep us in for an expansion or two.