the problem if you are really want build bank of bitcoin will needs good reputation,
unfortunately your are newbie and don't have reputation on here.
The people will not trusted in you with put their bitcoin on your service,
please don't offense it is the fact must be known.
There are lot of practical of complications on building a bitcoin bank and its reputation hence sticking with current system would be more convenient for many bitcoiners imho.
we still can not make loans if we are just put our money in bitcoin,
It is weakness if there are not bank of bitcoin although there is service like btcjam for making loans.
That is still not enough because of we must wait until there are people who gives the loans.