@PeachMcD Hostile NPCs would be easy to justify and your predator example is one good option. It is also easy to use NPC factions like goblins or invent something totally new like outcasts. If these invasions wants take to another level then players actions could have impact how often invasions will happen and how strong are those attacks. Meaning, depending how much players have for example killed certain mobs or animals or even if they have gathered too much a specific resources.
New World MMO will actually have some kind of invasion system and perhaps some ideas could be borrowed from there: New World: Invasions