The possibility to exchange items and gems would be great. I don't need 123456 legendary elixirs... really I'd gladly share with clan but I can't.
Same with gems, there is finite number of gems you really need, even aquamarines and others ultra rare.
Or maybe there should be option to sold gems for ingame gold. They are PRECIOUS stones after all, and were part of monetary system throughout the history.The fact that you say 123456 legendary elixirs would suggest that you dont have them.
You cant share because the game presently doesnt allow it, and it probably wont,because although it sounds a nice idea,it isnt a good one.
Selling them back to the bank is also a nice idea, but it isnt like there's a finite number of them,that they need to be recycled,so the bank doesnt need or want them.
If they did accept them as trade,what would you be happy with in exchange,that would also keep the bank sweet, when its they who want you to buy things?