I think that this will never happen, because the crypto currency is created to get rid of banks, that is, there is no point in this! In addition, the crypto currency means anonymity, so not many decide to entrust the disposal of their coins to the bank!
Yes currently its look like difficult, we even cannot think about it, but in future, when bitcoin will become more common, and more and more people will start using bitcoin, specially when government will accept bitcoin a legal currency, then may be banks will also start depositing bitcoin, because then bitcoin will get a very legal status, and will be the need of every person, but it is not so easy to happen.