Personally, the only form of trading i wish to see implemented would be the swapping of post-game loot. At the end of every mission or strike, everybody in the fireteam receives one or two items for completing certain tasks. Everyone in the team completed this challenge, and everyone receives loot. However, this is not always loot they want; it is just randomly given to each player from a pool of possible items. This can be a letdown to get items you don't want, even more-so when you can see clear as day that your teammate got an item you would prefer. therefore, I say you should be able to trade post-game loot with those who you played the mission with. It would be as simple as adding in a "swap loot" button on the post-game screen. You would select an item you want from a friend's loot, they would select one from your loot, and you would trade them off with one another. This still fits in with bungie's "earn your loot" mindset, as all of these items were gained by actions that you completed as a team. They were granted to each player for the same actions. However, it would allow for players to attempt to get items that they would prefer to have, rather than relying completely on luck of the draw then being left distressed that your teammate got the item you wanted for actions you did. I Think this kind of limited trading system could be implemented without changing the core aspects of the game, would not be game-breaking in the slightest, and would make many people happy.