Until we see otherwise we won't know about the universe. The in game direction is that it's all dead, even though that can't be true logistically speaking. But the issue is they don't want us thinking about it much beyond the promised "cosmic exploration." (Pretty sure it's gonna be Island Sanctuary in space)
Mysteries surrounding Reflections aren't a new mystery, hence there's no novelty in wondering about it. It's been on all our minds for years at this point.
What came before The Ascians could be interesting, but the knowledge that it will result in The Ascians inevitably would curtail enthusiasm, so it's something of a non-starter. (Plus the writers would have to set in concrete what sort of society lead to a perfection-adjacent society, and that'd be like opening a huge can of worms/would be really easy to fumble)
Because we are dealing with fiction, civilizations prior that have not yet been introduced will simultaneously insert new plot holes. As in, if they introduce... oh idk, let's make something up. Let's say they introduce a civilization that predates Allag or came right after Allag, and it wasn't quite as good but it was still pretty marvelous... We'll call them the Porxians. Anything they did will be just as important as anything they didn't do, and the mainstay of their Porxian civilization will have to be isolated to whatever 6 region map their expansion entails. Barring the topic not getting its own expansion, the lore surrounding the Porxians would have to explain why no one has ever heard of, mentioned, seen, or had reason to talk about them until the point of their introduction in our story.
This is actually what makes Shard civilizations so easy to write into the story, or really anything from the Interdimensional Rift. Inserting entire societies into a setting is not an easy feat, and explanations as to why they didn't come in sooner are usually extremely weak. Having a no rules dimensional barrier/passage is like a godsend to a writer in a setting like this. Isolationism and difficulty of traversal only go so far, after all, and when we've got civilizations capable of space travel even in Allag's day just sitting right beneath our pristine magical forest, it doesn't get any easier to hand wave away.
As for the, "half the world still to see" you really just mean Meracydia and the land beneath the Garlean cloud cover which has already been said to be typically impassable mountain ranges. I'm sounding like a broken record at this point, but the mystery there is also not new, and even once they finally turn us in that direction, the explanations as to why it didn't feature in our story yet will be weak, and the plausibility of its Source related portions being deeply meaningful to the setting will also not hold water.
I mean, the entire premise of the mystery in those regions is literally just, "You don't know what's there, for sure!" And the thing is, neither do the writers! And any mystery that is there is already hamstrung by the established setting and all that came before. (i.e. There is no escaping The Ascians on The Planet of The Ascians)