According to an interview with Ishiro Honda, this film was to feature "all of the monsters." Whether this means just more monsters than were in the final film or in fact every single monster that had appeared in Toho's films prior to that point is unknown. Honda also mentioned that the monsters on Monsterland would be bred and cross-bred with each other and fed by food cultivated in a giant underwater ranch near the island.
This film was the original draft for Destroy All Monsters, which was intended to be the last Godzilla film. Due to financial restraints, the film's monster roster and many elements of its plot were changed and cut down, and the film became Destroy All Monsters, with only the idea of Monsterland making it into the finished film.