Gender Neutral Options.

Seems to me the most elegant way to do this would be allowing sims to choose their own pronouns. I was really surprised when I started playing Sims 4 that they had thought of so many different specific custom gender options but hadn't provided an option for they/them pronouns.

Male/female frame could be put on a slider like the sliders we already have for body size and muscle development. That would be a pretty basic modeling operation in Blender using shapekey animation. Might cause some issues with clothing clipping depending on where you are in the slider, I guess.
Or the frame type could be like eyes and chins in CAS. There could be one basic frame instead of two, with a couple of presets pictured, perhaps "male" and "female", but the player can then edit shoulder breadth, pelvic width etc by clicking on those areas as we can already for eyes and chins.

And then, as with everything else about the townies, some players will want to live in a world that mixes their own creations with random weirdness, and other players will want to live in a world populated by carefully styled mannequins. Those players already know how to use cas.fulleditmode to change whatever they want about their townies.