Superman III: Supergirl

Producer Ilya Salkind originally wrote a treatment for the third installment from the Superman film series starring Christopher Reeve that expanded the film's scope to a cosmic scale, introducing the villains Brainiac and Mister Mxyzptlk, as well as Supergirl. The original outline featured a father–daughter relationship between Brainiac and Supergirl and a romance between Superman and Supergirl, even though the two are cousins in the comics. Warner Bros. rejected the outline and made their own Superman III film.

Why It Was Cancelled

 * The reasons are unknown.

Result

 * The third film became Superman III.