Trading items and implementing a system to keep weapons rare is simple depending on how you do it. Bungie could easily do something like this.. Players only 30 and above may initiate trading. Players may only trade once a week, or at least with a set amount of players per week. Players, like with xur, must trade either coins or mote for said transaction. So you're still working for it in a sense I don't think that would break the game. But It's terribly sad when my friend gets all my guns with amazing perk rolls and doesn't use them or breaks them down...like wtf!? Plenty have multiples , trading in this sense would be a good start.