The Dreaming is the realm of Dream -- for lack of a better analogy, his estate, kind of like Downton Abbey except with less Maggie Smith and more LSD. There are different parts of the estate that do different things and that are for different purposes, but it all belongs to Dream. While some of it does reflect thoughts and stories and dreams that individuals have had or have shared, generally it takes deliberate effort on the part of Dream of the Endless to bring anything into being in the Dreaming. When things have gotten out of the Dreaming, it's because Dream hasn't been paying attention for one reason or another.

Prophecy is generally the realm of Destiny, rather than Dream, so I'm not sure that using the Dreaming for canon plot would be the best idea in this particular case.