6. Roger Rabbit: The Toon Platoon
The Previous Movie: Robert Zemeckis' extraordinarily successful, groundbreaking hybrid of live-action and animation, centered around the titular cartoon rabbit and Eddie Valiant (Bob Hoskins), a boozed up private investigator, as they investigate a murder to try and clear Roger's name. Giving the cartoon aesthetic a gritty film noir gloss helped make this one of the most remarkable artistic feats of the 1980s.
Why It Was Abandoned: Technically a prequel, Roger Rabbit: The Toon Platoon was based around Roger's search for his parents, and the kidnapping of his wife, Jessica, though this eventually changed to become a "how Roger rose to fame" story. Steven Spielberg eventually quit the project, and with the budget flying out of control, it was eventually canned. However, as recently as late last year, Robert Zemeckis said that the script is done, though it ostensibly would not include Bob Hoskins due to his retirement, and is waiting for Disney executives to approve the written word. I wouldn't hold your breath, folks.