What I"d want there would never fit in the space provided...
When I was a kid, there was a terrifying-for-me-at-the-time-but-kick-@$$-in-retrospect haunted house walk-through attraction in Long Branch NJ. It has long since burned to the ground, and the surrounding area was pretty bad for a while (new developers, plus a local government with no problem ignoring the rule of eminent domain, is giving the area a comeback in the form of high-end luxury condos).
What I would love is a walk-through attraction similar to the haunted house concept, but with a Tomorrowland-Alien Encounter spin.
Have the building designed to look like a spaceship, the first (and, to date, only) spaceship to "Sling shot" around the sun, ala Star Trek 4. The ship is about to be decommissoned and turned into a space museum, when someone ( a CM dressed to look like a tourist) notices a long-dormant pod, or egg or...something in a dark corner.
Something bursts out of the pod and attaches itself to the person that found it. Medics take the guy away, but once he's in another room (there's a blind over the window, so we see it in silhouette), he gains superhuman strength, starts mutating, and bites the people who were trying to help him. From there, THEY become mutants, and your tour becomes a hunt for survival. The attraction then gives the illusion that OTHER tour groups who were before or after your group has fallen victim to the mutant alien virus. Victims with alien mutant DNA try to attack you from the rafters, or under your feet. The tour guide is responsible for your safety, but a few people are "lost" along the way (cast members who are able to slip in and play tourists who refuse to listen to the tour guide on how to stay alive). Eventually, someone realizes that, like Gramlins and Dracula & many a monster, sunlight kills these creatures. Here, the story has two options, depending on whether it is currently day or dusk. If it's day time, they pretend to open up sliding walls that hide windows, allowing sunlight to protect the innocent and destroy the mutants. If it's currently dark outside, the tour guide manipulates a series of sattelite "space mirrors" that can reflect sunlight directly on the museum. You are then able to escape through an "emergency exit" so you won't see all the charred remains of the other tour groups...
This is my pipe dream. It'll never happen, certainly not after considering Alien Encounter to be to terrifying. All the "no children or wusses allowed" warnings in the world would prepare the average family for something like that ("Come ON! It's DISNEY! How scary can it possibly be?!"). But I'd have no problem for something genuinely frightening to return to the Magic Kingdom. I'm sure Walt would agree that a man can dream.