If I remember LotR history correctly, Tolkien had a basic idea for the King of Gondor returning and the fall of Sauron, but he didn't work it into a story until the publishers, who wanted a sequel to the hobbit to make money with, liked his ideas along those lines. Then, through the writing out of the story, Tolkien invented many of its beloved subjects and events.

As for Star Wars, again if I'm not mistaken, it was originally pitched to 20th Century Fox by Lucas in a way where the toy sales would help to increase profits so the marketing existed in the franchise before the first movie was released. Plus, all movies are items meant to be marketed. Though I would guess that marketing was less important to Lucas than art in those days. Maybe.