Faction Based Game Mechanics
Think GTA

Clothing that has gang/faction 'colours'. Identifiers which could get you pegged even if an NPC doesn't 'recognise' your face.

Using the 'dispatcher' code to 'text' your faction's cells if you are jammed up...any available faction members get it, including NPCs, but NPCs only respond perhaps if they are within the faction zone. This would also be great for the double cross, decoys, etc.

PC factions that could recruit NPCs (gangs switch sides for example) if the faction reaches a certain watershed mark of members and activity in the game. Could be based on bribery for NPCs. NPCS recruited would need 'extra' care to retain them. Might be interesting times if extremely valuable NPCs become free-agent style.

I'll keep the thinking helmet on...

Also, NPCs will no longer buy items if you are disguised, they will tell you they want to see your face (this way we can compute ratings ICly and we can discern if someone is in a rival faction or has a bad faction rating ICly).

  I agree with the colors idea 100%. Regardeless of wether you're affiliated or not, if you wear the wrong colors down the wrong street in Compton, you will be viewed as one of the gang members whose colors you unknowingly sport, and you'll probably be attacked, beaten, robbed, maybe killed.  

  I second the vehicle thing too. If it's a vehicle that's identifiably yours, or you are seen parking it by a rival factions NPC, especially on their turf, something should happen to your vehicle.

  On an original note, how about gang signs, hand signals, or even gang calls, each specifically identifying a particular gang. I'm not sure how they could be implemented (emotes maybe??), but in effect they are ways to identify yourself as a faction member, or at least a sympathizer.  Not  all members of a faction will be able to recognize all other members, especially new ones or fringe members who don't publicly announce their affiliation. These signals could be ways to let them know you're on their side, or you could use them as a call for help if you shout them down the street and a fellow faction member hears it.

  I don't know a quick idea off the top of the skull there. I'll think about it some more and see what I come up with.

Also, if a faction dislikes you you may end up hated by them which will effect varios things though they will not attack you on site.

In addition to liking/disliking, I think it would be cool if there was a *fear* factor.

For example, the more you wreck a faction's shit, the more they dislike you - but also, the more they should be afraid of you, causing reactions such as leaving you alone, or running away (ideally towards other NPCs of that faction).

The chance of fear kicking in should depend on how many of the fearful faction are in a given room when the feared individual arrives. For example, if the feared character arrives in a room with one fearing NPC, there would be a higher chance of that NPC running away than if there were two, five or ten NPCs there. The "fear" effect could degrade over time as well.

It might also be cool to have "derivative" fear which would give a fear value to other members of the faction to which a feared character belongs.

I will use Judges as an example, because it's something I'm much too familiar with:

Say you have Judge Murphy and Judge Rookie. All Judges are hated by, say, Arteries. Now, there's a fight and Judge Murphy kills a given amount of gangers. The hate would increase (though I don't see how it could) but a fear value would also be attached to Judge Murphy, affecting Arteries. It might also affect other gangs to a lesser degree. Now, Judge Rookie would *also* benefit from a fraction of that fear, so when he's being dumb enough to take a stroll on Ashlin a single Artery might just give him a meaningful glare - whereas when he walks into a room with two or more Arteries he gets the shit kicked out of him.

If this faction stuff is still active, that is :P