One big problem with making the body mesh customizable (changing size and shape) is that it can interfere with clothing/armor and with cutscenes (causing clipping issues, etc.). Some body changes can also cause issues with how the mesh is rigged, causing weird visual errors during animation.
Slider-based customization of meshes can be a huge development cost, which is worthwhile for certain genres (like The Sims, where there are hundreds of procedurally generated characters, there's genetic inheritance, and customization is the whole focus of the game), but probably much less so for a turn-based isometric RPG.
I think this might be best achieved by offering great mod support (which I'm sure Larian plans to do, but the more the better), since it's a lot easier to tolerate visual bugs when it's just a mod rather than part of the base game, and it would mean anyone could make mods to perfectly suit their own tastes.
