Whether handled by skill trees or implemented as a standalone mechanic, factions should be able to grow in power.
Who? - Pretty much all organized groups -> gangs, mob, police, gorillas, soldiers, cannibals?. The decisions on development would be handled by group leaders (possibly the player)
How? - As I mentioned already it could be handled by a skill tree or a sub-menu (though I'd still opt for the skill tree layout). I think this could work very similarly to this: #1155770709432356875 message (I know it's also of my authorship, but why repeat myself?).
Long story short:
- Upgrades could be purchased with cash/ special skill points
- Some upgrades would have extra requirements (Want better guns? -> contact a supplier; Want to boost your comrades? -> solve their problems)
What kind of development? - Of all sensible kinds, but to list a few:
- Group buffs (e.g. Gangsters could be given Safe/ Confident In Crowds traits to make them more threatening in teams)
- Better armaments (Gangs again: Melee weapons only -> gradually increase use of pistols -> issue pistols to all gangsters -> go further? [shotguns, SMGs])
- More unique:
- Mobs could be upgraded to extort more money from businesses
- Cannibals could receive crit rate bonuses for axe hits.
- Favors: Imagine unlocking the ability to call in a van with gangsters ready to kick butt. And more...
And if you are not the leader? - Not everything is lost:
- You can ask your leader what they're up to... If you are more higher ranking, you can even try lobbying for certain upgrades: "Sir, I believe we should choose issuing body armor to cops instead of upgrading batons"
- You will receive a notification every time sth goes down at your faction ("Pistols for all" implemented.)
- At times you may be given a task activating the upgrade upon completion. ("Kill all soldiers in the armory to unlock SMGs for you and fellow mobsters")