The fact Xehanort has known about unreality for such a long time makes me wonder if he's actually been there in the past.
Young Xehanort was well aware the price for abusing the PoW was fading from this world (this reality), and Terra-Xehanort (or was it Master Xehanort?) asked time-traveling Sora if he, too, was ready to pay the ultimate price, implying Xehanort himself or someone he knows has abused that power before and faced the consequences. This has been theorised before on this site, if I'm not mistaken.
It could be the upper classmen from DR or Xehanort's classmates with the Norse god names who ended up in the other side, but that doesn't exclude the possibility that Xehanort experienced that fate and managed to return.
My point is that Quadratum as we have seen it is a city at night with a lot of skyscrapers. I don't know if someone has brought this up, probably yes because it's quite obvious and noticeable, but it's quite reminiscent of The World That Never Was. What do we call something that never was, something that does not exist? Non reality, fiction.
TWTNW is like an artificial world made by Xemnas, the nobody of a man who casually knows about non-reality. That can't be a coincidence.
Ironically, nobodies are but don't exist...whatever than means, because it never made much sense. They are incomplete beings, that's it. But if their hub world happened to be based after the image of another one from unreality or fiction which also doesn't exist (on the same side) that would be quite fitting and poetic.
The secret endings of the first KH was the first time we saw TWTNW, and the first Versus XIII trailer had the same vibes. And now the Versus concepts are seeing the light as part of KH. Maybe I'm reaching a lot and it's just Nomura recycling concepts again but it's not too farfetched that he will relate both locations. He likes making every little forgotten detail relevant, like the ark or the weird thoughts lately line from KH1.