I think I like these theories better than my own. They seem a lot more likely, especially since Madotsuki seems to be the only one who can walk around and do as she pleases.
My theory has the dream world as... well, just that, a separate world where people go when they're dreaming, and each of the doors leads to a new person's subconscious. The hub world can be accessed by anyone, and from there you can theoretically move around in other people's dreams, though a lot of people just stick around in their own.
Though not just anyone can get there (if they could, there'd be way more doors and the place would be overrun). All of the people who show up in the dream world have some sort of mental condition or inner turmoil, kind of like Madotsuki. I don't really know why Madotsuki is the only one who's able to explore, maybe she's just the only one who wants to... or is powerful enough, or something like that. (You can tell I haven't really thought that part through much.)
Poniko's world is there because of her subconscious desire to be isolated, and peaceful. That's why it's such a pretty place in the middle of a messed up world, and there's not even anything else there. I actually thought that Uboa could be some sort of "protector" (or even an effect) that Poniko uses to defend herself when something comes to disturb her peace. She must also like water, since both the pastel world and the inescapable blood world that she sends you to are full of it.
Monoko... well, expanding on what you guys said, she could have gotten into an accident in a tunnel, and is trapped there while her real body is in a coma. Monoe's presence could reflect her wandering around in a dark place, looking for her sister, but getting lost.
Masada is a piano teacher who wanted to be much more successful, which explains the spaceship. But he could never take the proper schooling, and thus had to abandon his real dream of going to space. The way the spaceship crashes could also reflect his dreams crashing down on him. ):