Back when Wikipedia started, the GFDL was a good choice for a copyleft license. The fiction wikia (Novelas) uses the GFDL, but it would be possible to switch to a different license. The alternative is the similar Creative Commons Attribution-ShareAlike license (CC-BY-SA).
If these licenses are very similar (and they are) then why bother switching? The creature to the right is an extinct arthropod, a member of the class Trilobita that existed for hundreds of millions of years then died out. In the ecosystem of the internet, survival times are typically much shorter…and the GFDL may be driven to extinction by the more robust Creative Commons family of copyleft licenses.
If you make the decision that you want your work of fiction to be freely shared with the world (free to copy, free to modify) then why not use the most popular license available for that purpose? As far as I can tell, CC-BY-SA is better supported and more widely used than the GFDL and that trend will only continue to grow. See: Forum:Change the license.