Before Disney acquired the Star Wars license, the canon universe was a gigantic beast, with countless comics, novels, video games and more all feeding into the same lore. While this abundant approach led to some of the most beloved characters and storylines in Star Wars history, it also led to some of the most bizarre, and it's no wonder Disney wanted to just start all over again. Depending on which side of the fan-base is asked, the Yuuzhan Vong certainly fits into one of these camps.
First appearing in the 1999 novel series Star Wars: The New Jedi Order, the Yuuzhan Vong are a powerful extra-galactic species that appears on the borders of the Unknown Regions, and proceeds to invade the entire Star Wars galaxy, wiping out over 300 trillion inhabitants in the process. Believing that mechanical inventions were blasphemous, these religious zealots were eventually stopped by Luke Skywalker and his academy of trained Jedi, including Han and Leia's children. While the Star Wars: The New Jedi Order series has some fun moments, the Yuuzhan Vong are one of the more divisive aspects of Star Wars Legends, and it's highly unlikely fans will ever see them again.
The Yuuzhan Vong Are Just Too Dark for Star Wars
When the original Star Wars trilogy wrapped up in 1983, George Lucas became a lot more willing to hand out the Star Wars license. Very soon, the Star Wars brand would be slapped on a sea of books, video games, and comics, all coming from a variety of different authors, illustrators, and developers. In the aftermath of Return of the Jedi, authors would turn to time periods not yet seen in the Star Wars universe, with many writers taking it on themselves to continue the story of Star Wars' iconic main cast.
One of the better examples of this is Timothy Zahn's Thrawn Trilogy, beginning with 1991's Heir to the Empire, and wrapping up with 1993's The Last Command. As far as the expanded universe goes, the Thrawn Trilogy is widely considered to be one of the better series, taking place nine years after Return of the Jedi, and chronicling Grand Admiral Thrawn's rise to power, and his attempt to rebuild the Empire in the wake of the Emperor's death. While this trilogy has some pretty out-of-left-field plot points, such as the Emperor clones, it's still a solid read with plenty of authentic Star Wars-y moments.
With the Thrawn Trilogy knocking the rest of the expanded universe out of the water, some writers decided to get a little weirder with their Star Wars pitches, and that's exactly what Del Rey publishing did with its Star Wars: The New Jedi Order series. Written by several writers, this series sees the Yuuzhan Vong appear at the edges of the known galaxy, and begin a widespread invasion of the New Republic. The Yuuzhan Vong, as a species, strongly believe in suffering and masochism, using painful organ grafting as a means to improve their physical prowess. Illustrators tend to depict the Yuuzhan Vong as Orc-like beings with spiked black armor and plenty of tendrils extending from it.
Put simply, the Yuuzhan Vong do not fit modern Star Wars. From a visual design perspective, the Yuuzhan Vong are much too overtly violent to fit in with Star Wars' universe, and from a tonal perspective, the Yuuzhan Vong are way off from Star Wars' usual vibe. While Star Wars can have some threatening characters and scenarios, there's a big difference between the horror of an organized Empire, and the horror of a genocide-minded civilization that worships pain and torture. The Yuuzhan Vong have always felt like they belong in the Warhammer 40,000 universe more so than Star Wars, and it's extremely unlikely fans will ever see them again, especially in a Disney-led property.