Egregores are thought forms created by a group mind. The thought form has so much energy pumped into it that it takes on a life of its own. These egregores can be useful in lending energy and focus to its followers. The important thing to remember is that with out followers pumping energy into it, it will basicly wither and die over time. This is why worship of the gods is so important.
Yes, in theory, an entire mythical world can be created. It is important to remember that this is an energy construct. You can't really go there. In fact, I would venture to say that there are countless energy "worlds" out there. Personally, I would lump into them Nirvana, the Kingdom of Heaven, Valhala, Gallefrey, Pern, Midkemia, Narnia and Chicago ala Jim Butcher. You go to them every time you read a book or watch a movie in an altered state of consciousness. Or, when you day dream. Or pray. Or... Well, you get the idea. Actually, since we are always creating this world around us, you can add this reality, this world, to the list.
Why would there be a limited amount of space? It kinda goes on forever. :)