I like how Minus Zero evokes Monster Zero, although I certainly don't think that's a hint that King Ghidorah will be in this.
I think the intended meaning of the title is akin to a rocket launch countdown. "T minus ten seconds, nine, eight...", you know? Calling this new movie Godzilla Minus Zero emphasizes that it's what comes next after Godzilla Minus One. It boldly suggests that Minus One was just the warm-up, the countdown to Minus Zero. The title declares that this movie is the main event.

