Building a central bitcoin bank means bitcoin being centralized so it is against the basic goal of bitcoin. Bitcoin has received immense popularity around the world because it is decentralized and paved the way for cross-border transactions with a minimum fees and without the consent from the regulatory agencies. So if it happens and if a central bitcoin bank starts regulating the bitcoin, the value will start decreasing slowly. Instead of creating a bitcoin bank, it will be good for bitcoin if the governments around the world start taxing on it and legalize the businesses build on bitcoin platform. That will be a very good move towards the main stream inclusion of bitcoin which will push its price to another level.