There’s a vast difference between a derivative work and a transformative work. In the case of this Seuss/Star Trek mash-up, the court held that it was not a parody, but instead did constitute a transformative work. Derivative work requires a license from the original copyright owner because it builds up and expands the original work, whereas a transformative work uses the original work in a new and new and creative manner. It’s rather like Pride and Prejudice and Zombies, or Sense and Sensibility and Sea Monsters in that sense. The original work is transformed, and not merely used as a starting point to boldly go where the original copyright holder never intended to go. In the mash-up referenced in the YouTube video, it was the current copyright holder of the Dr Seuss franchise who sued, and not the Star Trek franchise. Obviously, the Star Trek copyright holders are better versed in copyright law and fair use than the Seuss crowd.
AO3 calls everything on its site transformative, whether it truly is transformative or derivative in nature. However, you can’t merely label something transformative to make it so.
As an example, if someone writes a fan fiction based on the Harry Potter fandom, and creates a new work detailing everything that happened after Book 7, that is derivative. They would be using the canon work to serve as the launching point for their story, as they go on to tell us what happened to Harry and Hermione's children at Hogwarts. If JK Rowling were to object, I rather suspect she would prevail, because this work would, if published, infringe on any future novels JK Rowling might care to write about the next generation of Potters and Weasleys.
If someone wrote a fan fiction based on the Harry Potter fandom, and simply created a parallel adventure, or introduced a new character to interact with canon characters in familiar settings and events, that would be considered transformative, in that the new character might reveal feelings and motivations that would illuminate the author’s impressions of the canonical setting or event. It would not materially affect any earnings by JK Rowling from the canonical works or even future works, but it is still highly unlikely that any publisher would take the chance, without clear licensing, of publishing such a work. Legal defenses eat into the already thin profit margins of a publisher, after all.
And, in both scenarios, there is a limited portion of the work that could be labeled as original writing. That’s why we call it fan fiction, after all.