I'd like to see at least some of the minor factions as expansive, where it applies anyway. It'd be really cool if it worked something like this:
Location: Expansions must be located within 2 tiles of a mountain tile or be within 1 tile of a forest.
Diplomacy: No/Communicate via magic instead? Allows recruitment
Raids: within X tiles of nests
Diplomacy: Difficult/Magical Allows recruitment (one time)
Raids: within X tiles of nest
Expands: Moderate, capped at one level 3 city and up to 3 level 2 cities.
Location: Typically located near mountain ranges or elevated areas, also inhabit forests
Raids: Organized raiding parties target weaker villages in neighboring empires.
Diplomacy: Yes, Allows limited recruitment
Conquered cities remain of the goblin race and only have access to those improvements, size limitation remains in effect. Subjugation of the village allows limited recruitment, lesser quality than diplomatic relations. (larger quantities)
Expands: Light, small villages (level 2), typically only 1-2 of them per faction.
Location: Within forests
Raids: Defends home forest fiercely
Diplomacy: Yes, Allows recruitment
Conquered villages retain limitations and tech of the werewolves, lesser quality units than diplomacy creates ( larger quantities)
Expands: Never, main city is typically a fort or fortress, military improvement focused, troops are tough.
Raids: within X tiles of home city
Diplomacy: Yes, Allows recruitment, gold is best for gaining trust.
Conquered city tailored to produce units, both yours and a few set types available to the warlord originally, nothing super special though, maybe grants a champion/warlord NPC that submits once you conquer the city and joins your cause.
Expands: Often, Typically villages of level 1-2 with rare cities level 3-4. Troops are lightly armed and armored, move great distances at high speed.
Location: Barren terrain, often near oasis or other food improvements on the map.
Raids: Within X tiles of town, defends local resources viciously
Diplomacy: Yes, allows recruitment, may get better tech for greater benefits from food improvements.
Conquered cities produce lesser quality units, sometimes discover the tech improvements.
Raids: Within x tiles of tower
Diplomacy: Yes, usually requires magic technology, even offering of access to certain shards, necromancer will raise units for you, quality increases with better negotiations.
Conquered tower is destroyed, perhaps they can be built on a site of magic power so the conquering nation can take control of it after victory.
These are just examples, races and whatnot can obviously be altered to fit into elemental lore, the idea was to give a baseline of the different kinds of "minor factions" there could be and the behaviors that I think would be neat to see. I think it would go a long way to making the world feel more fluid and alive, especially on larger maps where you might not run into another major faction for awhile, it'll add some extra flare to the early game and give you things to deal with in times of peace later on.