This actually pertains quite a bit to what I write. I had a story hour up for a while following a mix of my own creative writing and Play by Post material (See Sig - A New Power), which tanked after a bit and the last update is me finished, having covered all game material and make a quick update using just my creative material to get back to that after having worked with other people for a few months. I'm actually planning on getting some more writing done tonight, so I may have an update for that one soon.
It's essentially taking an idea I had for several characters, specifically two atypical Jedi showing up around the same time as Star Wars Episode 2 and giving it a go in writing. There was a post or two with just creative introductory material, then it zooms forward several decades to post Episode 6 shortly after the events in the movie. At this point the players come into play and it becomes a recording of a PBP game. There is some loose interaction with the core story of Star Wars and a few of its characters at points, largely as flashbacks between the PBP era and events of my own two characters.
Lots of lightsaber fighting, new philosophies on the force and politics, and for a lengthy span our heroes are repairing their ships which have crashlanded on a planet inhabited seemingly exclusively by giant vegetation and voractious flesh-eaters. This in top of landing on top of a new order of Force Users who have discarded the teachings of both Sith and Jedi and are beginning their make own presence felt in the Galaxy. All the while a Rebel operative and a very strong-willed droid are sent on a covert mission in response to Rebel Intelligence spotting our heroes' distress signal, in the middle of previously uncharted space several light years from the Outer Rim. Plus, the Empire caught wind of the same signal and are on their way to claim new territory, and 'remove' anyone who gets in their way.
As far as just making something up, I just take an existing world and give it my own touch, though I have been developing an entirely new world based off of my own ideas. Gonna be a while before there's enough there to write something with.