My KH4 story direction theory:
Keep in mind that Missing Link got cancelled so none of the following information is canon. In Missing Link the astral plane was described as a world made up of mental energies. It was also said that Heartless come from there.
[quote]Freya: How can I put this... In essence, the astral plane is a world made up of mental energies. It may look real, but it's a strange world where no one lives and about which we know little. It's connected to our own world via corridors we call distortions. The astral plane is also where the Heartless—those heart-stealing monsters—come from. This is simply speculation, but it's believed that they come here because there aren't any hearts for them to steal over there.
Now the KH4 trailer talks about that the appearance of our characters brought these ideas and concepts into Quadratum. As the last story arc showed there isn't really a way to completely erase light and darkness because they are natural forces in the KH universe. Xehanort's goal pretty much was to achieve perfect balance between light and darkness by resetting the universe but his plan had the issue that he still has to abide to the laws of the universe.
In Union X it was said that the Master of Masters plan is to eradicate all the true darknesses but even with his plan darkness can't be entirely erased due to it being a concept of the universe and he would end up the same way as Xehanort. (It also makes me wonder if Xehanort got the actual idea of his plan through the Master of Masters)
Now after watching the trailer it makes me wonder if sealing the darknesses in the foretellers is just a way to prevent them from acting until the Master of Masters has figured out a better plan which is the actual deletion of the idea that light, darkness and other in-universe concepts exist.