The result of my boredom today... XD
This is a very strategical slider, this means that a range of 5%~10% is enough to achieve a "normal" depth into the forehead.
The body skeleton doesn't have any forehead bone, this means that hair meshes may look out of place if they show the root really near the scalp area.
And the only way to do this so far is by a GEOM slider as there are no forehead bone in the Sim structure.
I did not touched the near-scalp vertices, for obvious reasons. XD
The slider is not bidirectional, but I may make it so in future.
It works for all ages (patience... XD) and for both male and female.
I made GIFs so you can see how it affects. The blue is the original morph border related to this morph.
Next step would be to make a brow depth slider, so we can have even more control how the forehead looks overall.