Probably *extremely* hard. Remember that they're already taking into account 3 dimensional expansion for most parts, but what we DON'T have are "muscle sliders" and any distinction between upper and lower body / arm / leg thickness.
I'd settle for the ability to bulk out arms or legs, abdomen and the like separately, because at least most of the work's already been done. It might have the same code but they'd need to "stop at this area" on the body, "start at this area" for arms/legs.
A primary concern is how this would all affect clipping issues, and while there are some small issues allowed (the valk cape and wings for instance) it's of primary concern to the animation and art crew that things look *excellent*. All costume parts would have to be tested on all three body forms, and adjusted if possible for serious issues.
I think that'd be way more work than necessary, really.
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Repurposed
How difficult would it be to modify the body sliders to go from a single slider that covers all three 'dimensions', to three sliders, one each for the X, Y, and Z axes of the particular component, like the face sliders?