What's your opinion of 'Common Universes'

Places that have been franchised out for interested authors to continue writing in, (e.g., Niven's Known Universe, specifically Kzinti stories, and Laumer's Bolo stories,) or universes specifically created to be shared by authors like Robert Aspirin's Thieve's World.

Obviously Star Trek and Star Wars are obvious examples of the same thing.

I wonder what it would take to create something like this for our little kink.