With so much focus these days on sculpting for 3d printing, a very useful feature or plugin would be dynamic lighting or materials. What I mean is for zbrush to lock the lighting axis to the subtool, or material with live updating. What this would do is allow a two tone material or specific lighting to identify undercuts in sculpts for plastic production or mould making.
the current method that can be used in zbrush is using polygroup front and looking for undercuts that way. another is to have a matcap with two colors split down the middle and keep returning to a predefined view.
Additionally, a plugin that works with back filling undercuts with defined draft angles would also be great.
There is an old thread that touches on this, here: