So I've been thinking, there are parts of Equestria that are literally "monster zones", case and point - the Arimaspi, giant cyclopean goat people.
It would be good idea to add them as a faction that's based on border skirmishes and after reaching a border of other monster zones, either force those zones into an alliance or outright conquer them.
And to make it even more interesting, but as an optional path/decision, have the arimaspi search for Grogar's Bewitching Bell and then use it to absorb magic through conquest, so they can then use it for the ritual to bring back Grogar, the First Emperor, the FATHER OF MONSTERS.