The only way to make it expand forever would be to support recursion. Would be interesting what texts you could come up with that. Kind of a textual Mandelbrot set.
It was a dark and stormy night. The captain and his men were huddled around a campfire. The captain turned to Jake and said "Jake, tell us a story.", so Jake began his story: "It was a dark and stormy night. The captain and his men were huddled around the campfire..."
Perhaps using some kind of Markov chain process? It could be amusing, but would be more interesting with some kind of understanding of parts of speech.