Ummm... Ooops

How can the dilemma (or any one, specific dilemma) be an integral part of the reader-created narrative?

Well, (cough cough) like any successful fictional work, Myst is not wholly user-controlled. How could it be? Such an undertaking would be impossible, as it would require the creation of an entire world, with an infinite number of possible story lines available to the reader.

Myst comes pretty close.