On a side note, just for fun, Tezuka and the writers of GMMG came up with a mini story idea for Godzilla's 50th anniversary movie. Something like a "Destroy All Mecha-Monsters" concept, in which humanity rebuilt Mechagodzilla as well as an army of other mecha kaiju, which subsequently ran amok. They drew two storyboard drawings depicting the conceptual idea:


You'll notice Mechagodzilla in those pictures, along with concepts for Mecha-Baragon, Mecha-Kumonga, Mecha-Gaira, Mecha-Sanda, Mecha-Varan, and Mecha-Kamoebas, among others.
But this idea was never intended by any Toho executives to be the 50th anniversary Godzilla movie. This was just something that the writers of GMMG thought up for fun. Indeed, Ryuhei Kitamura, Wataru Mimura, and the other writers for Godzilla: Final Wars were already underway on writing the screenplay for that movie at the time of GMMG's release.