This would require quite a bit of work, but I think having a personal place to customize and live in would be nice. This should definitely include all the necessities including (but not limited to):
-A Mailbox
-Ammo Resupply
-Armory
-A nice view of the city
-Firing Range(?)
For an obvious example of this, look to GTA. As an additional 'expansion', clans could have their own quarters, possibly with a clan-only vendor where one can buy/unlock certain items you can't get without a clan.