Alright. So, here is a chart of what I think the gender differences the monsters can have, feel free to add an idea in the comments.
Godzilla-Male: Looks like how we all know him.
Godzilla-Species-Female: Probably a lighter shade of grey than the G-man, with rounder spikes.
Anguirus-Male: Anyone of his incarnations.
Anguirus-Female: I imagined the late Shows era version with darker skin, and maybe a different carapace.
Rodan Species: We know that they both look the same.
Varan-Male: Just like he appeared in his debut film.
Varan-Female: Maybe a lighter color with the horns arranged differently.
Mothra Species: Either we get a Mothra, a Leo, or a Battra. Leo seems more possible for the male.
Well that's all I can think of as of now, so I'll catch you guys later.