The first would have pit him against The Human Vapor, a character from an earlier Toho scifi film. The production never got very far past the initial draft stage.
The second attempt (from which some elements would be used for this film), would have pit him against Godzilla. Toho planned on making the film as a follow up to King Kong vs. Godzilla (though a script was actually writen before that particular film was made, with Godzilla awakening from that iceberg he was trapped in). The script was dropped all together due to some very confusing plot points. Most infamously, the fact that the JSDF would have crossed the Godzilla Threshold by freeing Godzilla from said iceberg themselves to kill Frankenstein because they are afraid Frankenstein will start eating humans.