Hi, I'm a newbie and I'm not familiar with mmorpg or coding. I have two ideas though.
1.Profit in banks
Banks should give some profit from saving money

Because bank is already treating all acount chars as one, it shouldn't cause making false account etc., and it would be fun xD
2.Commission sale
You probably all know situation, when you try to sell something, but nobody is interested, even if price is very low. Instead of talking or exping, you sit, send "sell scythe 5k""sell scythe 5k", when maybe someone who is interested in buying is somewhere deep in a cave

I think that there would be good, if commision sale is made. You will give an item there, set price and check sometime if someone bought it. It wouldn't destroy normal trading at all, because when buying rares, exchanging or selling a pack of items negotiating is irreplaceable.
Humble 5xlvl.