2. Haunted Mansion (Disneyland)

What it could have been: The facade of the Haunted Mansion began construction in 1962, and was completed in 1963 - but the ride itself did not open until 1969. Initially, the building was expected to contain a walkthrough attraction, along with a pre-show/queuing area and a themed restaurant. The pre-show area and restaurant were both to be themed around a "Museum of the Weird", packed with curiosities "discovered" all over the world.

The decision: After Walt Disney died in 1966, Disney's Imagineers decide to radically alter the Haunted Mansion concept. Instead of a walkthrough, it became an Omnimover-based dark ride. The Museum of the Weird concept was dropped, although it has since been revived in comic book form by Disney-owned Marvel.

The result: A stunningly detailed dark ride through a haunted abode that has entertained generations of visitors to Disneyland. The fingerprints of the Haunted Mansion can be seen on attractions all over the world, from Disney's newer creations such as Mystic Manor at Hong Kong Disneyland to likes of Duel: The Haunted House Strikes Back! at Alton Towers in the UK.

1. Harry Potter and the Forbidden Journey (Islands of Adventure)

What it could have been:As early as 2004, Canadian firm Dynamic Structures – which had supplied the track for Spider-Man – was showing off a unique new ride concept at the IAAPA trade show in Orlando. This employed a RoboCoaster, a new type of simulator based on the same technology as robotic arms used in manufacturing and designed by German company KUKA, mounted on a track. Universal planned to use this to create an attraction based on 2004's action horror movie Van Helsing. The ride would have incorporated a variety of special effects as well as projection screens.

The decision: After the huge success of the first few Harry Potter books and movies, it was inevitable that rumors would begin to circulate that theme park operators would seek to license the characters for use in rides and shows. Realistically, only Disney and Universal had the money to do J.K. Rowling's creations justice. Disney came close to snagging the rights for Potter, getting as far as signing a letter of intent with Rowling. But it walked away when Rowling demanded too much creative control over the project. Universal was only too happy to pick up the pieces - and it dropped the Van Helsing concept in favor of using the ride system to create a "flying" ride set in the Potter universe.

The result: A unique dark ride that employs robotic arm technology to take guests on a simulated "flying" adventure with Harry Potter and his friends. The Wizarding World as a whole, anchored by Harry Potter and the Forbidden Journey, has transformed the fortunes of the Universal Orlando Resort and clones are on the way to Universal Studios Hollywood and Universal Studios Japan.