Add a Trading System to D2!:
You could even lower the light of traded Loot to prevent people from breaking the game or trying to power level each other.
You could prevent Exotics or Loot from Quests from being traded so that the game doesn't get broken.
But imagine if you got a god role Sniper that you'll never use while a friend had a god role shotgun? You could trade each other for the other weapon and it could improve the game over all.
If resources or currencies could be traded, except for Silver.., it could create a community economy in game that could aid in immersion and clans could come to mean some thing much more than just a group of friends who play together. You could then add things such as a Clan shared Vault.
Edit:
And to improve upon my idea,
Make it so that a player must reach a certain Light Level to trade so that Noobs don't try to get an All Legendary God Role loadout right out the grave.