First problem is to start your own bank needed to permission and there will question asked by them that his will you grow your money , if you will say that it is only bitcoin for which we start bank than what will be your reply for the down of the price of bitcoin .
In this way you can't prove that you can start bank with bitcoin . Instead of start bank you should make own money investment the bitcoin and trade like a fiat bank money .
Well here you need to think that idea of bitcoin bank already is looking to start own service but I am.not sure what will happen in future .