I'd like to interject and put forward that many people don't understand the "FAN" mindset. Love for an established character/franchiseso strong it could would drive people to make derivative works based on something already firmly established.
I'd like to point to Paul S. Kemp, Alex Irvine, Drew Karpyshyn, Dave Wolverton, Jude Watson, Ryder Windham, James Luceno, Michael Reaves and even Terry Brooks have all made a pretty penny writing Star Wars Novels (Terry Brooks even had to adapt a screenplay to novel form, so even less "Creative License").
Creating a story in familiar settings has been a staple of the low/no budget writer's credo from day one and is stated repeatedly not just on this forum, but on many others and by successful Hollywood Industry and Independent Filmmakers. If you are supposed to "write what you know" and you know Godzilla, specific comicbook characters or movie franchise worlds... write it.
I'd even go so far as to say shoot it, but know that you won't be able to sell the finished project. However, it will give you a framework upon which to build your filmmaking chops and lead to better productions on the stuff you can sell (although, you can probably sell the script to the franchise owners if it's written well enough - and registered at the WGA and Copyright offices).
