You can also write stories set in a collaboratively-created world. I think this is commonly called 'conworlding', but I don't know enough about it to know if I'm actually using the term correctly.

Here are two examples of the kind of thing I'm talking about. (If you sign up for the former, which has a rather high proportion of transhumanists playing, let them know that Adelene sent you - I get imaginary currency for recruiting.)