A bitcoin bank will be very bad because than it gets more controllable and that is very bad because we want to control our own money right, I think its better do hold it like this without a bitcoin bank otherwise it also getting less more secure if you ask me.
My conclusion is that if there comes a bank it will destroy the bitcoin.
I think Coinbase is more or less like a bank and include many functions in one place. This is the type of services bitcoiners are looking for.