Instead of picturing factions as a whole try to think of them from a recruit's point of view and ask yourself "what does this official faction offer over an unofficial one?" then go a step further and ask "Why should I join a faction over remaining solo?"
You could break it down into the faction groups and see what rewards would work best for each. Perhaps police factions would like a heavy fighter patrol craft thats slightly faster in/charging cruise? Would the Navy factions like a custom fighter skin with kill counts on the side (not individual of course, think WW2 fighter pilot markings)?
Think small changes that effect everyone and I'm sure you'll come up with some interesting ideas.
