Actually in my country bank already works with crypto or rather online wallets, so what happens is that they give features to the wallets to sell their bitcoin to cash it out using banks or atm, by doing this it solves the problem of fluctuation. This is the best way to approach cryptocurrencies as a bank since banks are too busy with stock markets already, and since everything can be accessed online with cryptocurrencies including exchanges going to banks isn't necessarily required so meaning banks has no way of working with cryptocurrencies unless they create projects of their own or create an exchange using their trademarks.