Well I never thought about the Dream World as something else, what did you think about it ? For me it always was kind of place where every mages can meet in spirit during their dreams (if they are close in reality, because it seems like the more distant they are in reality, harder it is for them to contact each other if they can't travel safely through the sea like Reynir). And the troll spirits seems to have access to this place as well (for example the dog in the first adventure). I also wonder if the big aquatic serpents who tried to drown Lalli are trolls or something else because we never meet them in the reality...
I really like the idea of interstellar clouds of magic energy, these could also explain why the magic "disappeared" from the world, and they could also be at the origin of the illness!
About the artifact theory, did you imagine that a bunch of people found it, got infected and became the first ones to contract the rash ?
But yes, there is so many stories or theories we could write about it ! I really hope we will get answers to theses questions as the story goes on...
I see two possibilities for the Dream World (with many variations...):
a) The Dream World is that, a Dream, but a shared dream, accessible by telepathic power.
b) The Dream World is that, a World, but in another dimension, accessible by magic power.
In both options we need to switch off from the real world to gain access.
In a) the mind connects with the "dream internet", but all "data processing" happens inside the minds of those connected. If everybody was simultaneously awaken you would not be able to connect and will remain on your dream inside your mind.
In b) the conscience, while still attached to the body, enters another world/dimension, that exists by itself. Keeping with the computer analogy, the "processing" happens, mostly, in a big server. Even if everybody else in the world is awaken you can still go inside.
That last option is somehow supported by that moment when Reynir (someone gave him admin powers but forgot to tell
) enters on Lalli dream world while Lalli is trapped in Emil's mind.
The big serpent can be something that inhabits just that world, or the projection of the mind of something very big (it seems that minds retain a large similarity with their real world bodies, we never saw someone turning in an animal or changing size, except for the children versions on Lalli's island dream/memory). So it could be the mind of a sea-serpent? Minna already draw big sea serpents a few times. I recall the ones under the ship on page 55 (adv 1) and on page 31 (adv 2). So maybe that's it?
I also love the interstellar cloud theory and will, with time, try to write something using it.
On the artifact hypothesis it could be either your approach (the artifact contains the infectious agent) or the artifact unlocks a connection with another dimension, a gate through which one, or several, evil entities access our world and their actions cause the disease.
As you said, many stories could be made. I'm also very curious about Minna's answers to all (or part of) that, if we ever get some. Will Minna ever write her "Silmarillion"?