Xcom2 spawning areas (red flares) were good. I liked how they appeared in random places (still nearby though) and, important, appeared unexpectedly. Sometimes it turned the tide. I think something similar can be done in PP. Some creatures could come from beneath the ground (like X2 chryssalids), but in the manner of that red flare reinforcements. Something like… you see the ground is trembling in a certain area… something is coming… but you don’t know what… -)
Current static 1x1 spawn zones look cheap and artificial. Maybe old xcom exit/entrance areas (on the side of the map) feature could work. Somehow mark the sides of the map from where enemies may come, and not with 1x1 tile, but with a longer line. Maybe even mark the whole side of the map - like, well, enemies may come from the east, but no one can say where they will appear exactly.