It was brought up, but Larian rejected it.
I am all for it, but not "random" more that in any situation where a vacuum exists something will come and fill that vacuum. In this case after a given number of game days areas that were under the control of X creature might be filled with any number of a list of Y creatures that fit the ecosystem.
Caves, swamps, ruined towns, abandoned monasteries - all areas that could attract a subset of various types of creatures.