Not all #fanfic is a shallow copy of the original work or a continuation of it. Good fanfic usually explores concepts the original work was afraid to, or begins from a premise in which the original work could never have occurred while retaining some elements (settings, characters, etc) that still make it relatable to the original work.
In many cases, #fanfic is specifically a reaction to popular but trivialising media in which the fic creators attempt to reclaim genre themes which have been discarded in favour of saleability.