There are still constraints, and I specifically mean constraints in terms of different aspects of the theme and what one can do with it, in terms of creating attractions (rides, shows, etc.), restaurants, and shops. There's less diversity with a land based on a film. Star Wars is a series of films written by George Lucas and a few others. There's only so much you can do with that. What's written in those seven films is what you've got to work with. Now take a land themed to, let's say, Los Angeles... You've got plenty to work with due to history alone, let alone culture, landmarks, etc. You've got multiple aspects of Los Angeles you could work with, film and television, car culture, Los Angeles noir, the Mexican influence in Los Angeles, the fast food culture, etc. These aspects of Los Angeles are all significantly different from each other, and would create a diverse land.
It's like comparing a land based on Disney's Hercules to a land themed to ancient Greece. Which one are you going to get the most out of, and lends itself to some actual room for creativity?