“The Dream itself is said to be made of aether and memories, and is capable of showing not only the past and present, but also possible futures.” Very much like the fractals.
The dragons themselves seem incapable or unwilling of accessing the mists without corrupting an intermediary. As Mordremoth’s facet of corruption is mainly mental it would make sense that a sylvari with a mind partially stored out of bounds would be resistant to his influence.
The dream has to be in the mists because all things are. But because it is does not seem to be located in tyria itself, a fractal seems to make sense.
Edit-adding: “the Pale Tree is capable of sending beings, including non-sylvari, into the Dream as well, through a portal similar to those which often lead to the Mists.” source for quotes is https://wiki.guildwars2.com/wiki/Dream_of_Dreams
“Kentigem”-chief. Born cycle of Dusk. Wyld Hunt:
Learn as much mending and medical info as possible so that it can be added to the Dream.
Become the first Chief of Mending and guide the newly awaken as well as those who want to learn.
The Mists is the building blocks of reality itself. Surrounding Tyria is the Mists and the only difference is that Tyria is ‘stable’ parts of the Mists. The exact mechanism of how the Mists comes to shape matter is unknown, but what we do know is that it can take the form of past events (as seen in the Fractals) or even alternate events (as seen in the Asura PS) or just random stuff. Fractals are more like a echo of sorts, an imitation of something that is or was.
As for the Dream… we don’t know if it has the same tangible properties. We know the dream is a mental phenomena (the Sylvari are connected to the Dream and their experiences are stored in the Dream i.e. why we were able to see Caithe’s past). But so far we don’t know if the Dream can shape tangible realities like the Mists (every time we go into the Dream are we physically going in or simply going on a out of body experience?).
So the primary thing that connects the Mists and the Dream is this capacity of reflecting past and future events (not sure about future events and the Mists though). But were the Mists seemingly randomly forms fractals the Dream might be more purposeful? Maybe not. We can assume some purpose, i.e. the Wyld Hunts are seemingly an attempt of the Dream to achieve something. But maybe the Dream also just does things because that’s how it works.