The Simpsons are well-known for their spins on TV shows and films. In fact, any parody or reference made on the long-running series is considered an honorable compliment. It's what The Simpsons do, and by golly, they're good at it. To completely list all the franchises, media, and pop culture references the show has done in its 35 years would take another 35 years, and there's no time for that.
When brainstorming what hasn't been parodied yet into an entire episode, only a few shows haven't been done. Yes, in all fairness, they've been referenced, or the audience made their own connections to them while watching. I have noticed threads of popular shows that haven't been made into a full episode woven into the Simpson lore. Whether it was unintentional or not, or if it is, it could be an Easter Egg, a foreshadowing of future storylines, or an idea that didn't get the green light.
The productions are quite popular themselves and have already a strong fanbase. However, if it's out there somewhere in the universe, chances are it'll one day get The Simpsons' treatment.
3 shows that haven't been parodied yet on The Simpsons
The 100
The 100 is a post-apocalyptic survival drama that aired on the CW Network from 2014 to 2020. It follows the story of Clarke Griffin (Eliza Taylor, who was sent down to Earth along with 99 plus one unauthorized juvenile prisoners to see if the planet was habitable. Clarke became the elected leader of the group and experienced things that were both heartbreaking and joyous. The drama is one of my favorite shows and is equally vital to The Simpsons.
Kass Morgan authored the four-book series of The100, and other than the three main characters (Bellamy Blake, Octavia Blake, and Clarke), the storylines are quite different from those in the show.
While The 100 has been referenced in season 30's episode 8 in its full name, The Hundred, on a TV screen, and I picked up specific cues in the "Thanksgiving of Horror" episode in the segment, "The Lst Thanksgiving," it hasn't been created into a full episode or a "Treehouse of Horror" story.
The way that I'd imagined the parody would be Lisa helping Raven Reyes and Bart being friends with Bellamy and John Murphy. As The 100 is an emotionally complexed and intense show, incorperating humor into its stoic narratives will be challenging, but not impossible.
What would make it even better is to have some of The 100 talents guest star for the episode.
Scooby-Doo
The dog and his gang of friends solving mysteries in a Volkswagon van has been referenced a few times, and we're waiting until the Simpsons characters are molded into the forms of Daphne, Fred, Scooby, Shaggy, and Velma. It'd be iconic, as you'd infuse two of the most notable animations into one. The reveal would be a character you'd least expect, and it has to be a superior mystery, one that the often inept Springfield PD fails to solve.
Santa's Little Helper would make a great Scooby Doo, seeing how he has done work as a police dog.
The Wizard of Oz (and Wicked)
It has us questioning why the writers never utilized The Wizard of Oz or Wicked into an episode. I'm positive that the classic movie and upcoming film has been referenced in episodes, but having a full episode dedicated to both would be a simple walk down The Yellow Brick Road.
There are a ton of fun scenarios that would make the parody funny and keep on point with the original. For the wants from the Wizard, Homer would want a beer and a donut; Marge would want a moment of clarity; Lisa would want a Harvard scholarship; Bart would want an idea book about antagonizing Skinner more; and Maggie would want a new pacifier. There are just so many directions this possibility could go.
These three shows-to-parodies could all be embedded in a "Treehouse of Horror" episode, seeing how they don't canonically connect with The Simpsons but would work well for the haunting franchise.