[Demisis_P2P]

Just modify parties.
Remove the word Party from them.
Then make it so parties have levels, and each level you gain allows you to add another member to your party. Parties get exp for killing other players, holding capture bases and for winning events.
When a party gets to level 10 (15 members) they can buy a base for X amount of money.
Over time gangs will emerge on their own and buy up the bases.

Then you just need some way for gangs to lose control of their bases if a better party comes along and wants it (captured for 24 hours straight?).