Unmade Daikaiju Movies

Here's an odd bit of trivia: King Kong verses Godzilla originally started out as a proposed sequel to the original Kong film wherein the titular ape battles a gigantic version of Frankenstein’s Monster. Although American studios showed marginal interest in the idea, it was the Japanese film company Toho who eventually purchased the script and then reworked the story, most notably replacing Kong's intended opponent with their own creation.

This site categorizes a list of giant monster movies (with accompanying info) that never achieved actual, finished production. While it’s not exactly a bad thing that a couple of these films disappeared into cinematic limbo (I’m looking at you, Crackodile) there are more than a handful of aborted projects mentioned that sounded pretty entertaining.