Allowing players to join an npc faction.
Their Faction won’t attack them.
Helping their faction/Fighting other factions raises their reputation, allowing them to get (Possibly unique) faction items or gain a follower/thrall at the cost of reputation points.
The player would be able to spawn at a faction based spawnpoint.
Infighting or behavior that your faction does not approve of would lower your reputation points.
Additional:
-NPC patrols around faction’s main area.
-Faction wars/relations. (Some neutrals become enemies or friends)
-Faction skirmishes in the wild.
-Faction quests to join and get reputation.
-Faction based side stories/characters like Conan.