Why not just base the Multiplayer on Co-op and have say teams of 1-10 people upto 10 total Players in the game(or per server/realm w.e). Make a lobby and/or allow to have pre-made teams join/create a realm.
Allow Teams to be named/created/changed in and out of game and joined IN GAME via some Loyalty Mission for the team. Custom missions and such, or a from a list of pre-made "Loyalty" missions for the team leader/or team members to vote on and such. In Game Chat optional.
And just use different modes like:
Gang vs Gang = 2 Teams of up to 5 players.
Us VS The World= 5 Teams of 2 players.
One Man Army= 10 Teams of 1 player.
Once Upon A Time= 1 Host(Storyteller/Dungeon Master of sorts) AND 3 teams of 3 players. Or 9 Teams of 1 player, 4 Teams of 2 Players or 2 teams of 4 players and one "Rogue"(Neutral and/or boss) player.
Co-op a Feel= Any amount of teams of 1-8 Players(Min/max of players per team 2-8).
And so on...
I like Co-op in Saints Row series the most. It's the best I've seen, why not advance it further?