While this has probably been posted before i believe a player to play trade system would be an amazing option and would be worthwhile, even if its only between guild members. You could add a tax system or make it so gold isnt tradable. It wouldnt truely affect the Central Market that way and since some items arent really sale worthy other guild members might want or need them.

Real money trading (RMT) is real and has killed many games. Trading while it would be nice, would surely kill this game. I would like a marriage type feature that binds 2 accounts permanently. But even that has potential for being exploited, even if just once.