Well, early on, fans of pulp magazines and early radio shows were among the first to dabble in what could be considered fanfiction. However, in more modern times, fans of shows like Star Trek really led the way. Gene Roddenberry's creation had a huge and passionate fan base. Those early Star Trek fans who wrote stories set in that universe were influencers. They showed that it was possible to create engaging stories within an established fictional world, and their work inspired other fandoms to follow suit.