Jiggle Physics need be the Next Expansion Key Feature

[Absurdity]

All female characters outside of roegadyn (vomit) and bunnies are built like vietcong guerrilla fighters who lived all their life on rice with 0 feminine assets so there is nothing for them to jiggle.

Fem Miqo butt is also Hank Hill-tier.

Let's be honest, our character proportions are a clown fiesta in a few more places. Spindly arms and legs, nonexistent hips and a seemingly really wide waist because of it.
...And don't even get me started on Elezen proportions.

Except with Customize+ you can increase your butt size....

From what I understand that plugin just lets you scale the bones and this warps whatever parts of the model are rigged to that bone, it's the same technique the devs use to achieve different-looking bodies on, for example, female roegadyn compared to female miqo'te despite both using the same base model.

Looking at standard XIV models in something like Blender or 3DS Max shows you actually can't...at least not really.
The standard vanilla body has a hip bone but no butt bones, the butt is simply rigged to the hips and scaling the hips inadvertently also increases/decreases the butt.

If they wanted to add a butt slider they would need to add those bones to the character skeleton and redo the rigging on every piece of chest (that goes below the waist) and leg gear.