I have an idea. In this modification you can include something like gang wars. On start you select your own gang and have your own "soldiers"
For example you are Mafia boss and you are in your limousine with your bodyguards (NPC) and chauffeur (non-npc, or if he will be NPC this be good too
but I know that will be hard to make it). Bodyguards in your gang cars following you in limousine (everybody NPC, driver too) will be very nice. If you can do this in Liberty Unleashed this multiplayer mod be the best of anything multi mods for any GTA