Of course Musk wants Twitter to be a bank.  He's already been pretty vocal about wanting Twitter to be like WeChat and be the only app you need for everything.  Fun fact about WeChat... you say something bad about the Chinese government or repost pictures of protests, they can cut off your access to the app which cuts off access to your money through the app which means you can't do things like use it to pay for the bus/taxis/trains to get to work.  And that's what Elon wants for Twitter.