Let's face it 'Assuming everyone tomorrow agrees to standardize that Bitcoin is the de-facto currency and decides to transact in BTC only' is a crazy assumption right now.
err consider John Doe... He doesn't give a damn if his bank is open source or not. John Doe is not your target, people who use BitCoins (and I don't mean lemonade selling girl scouts) are your target.
Electricity, Air travel, Internet? Don't you think these were crazy ideas some day?
With Bitcoin ATM's opening up in Toronto where I live, I see things moving fast than even myself expected. Sure, there will be resistance but it's hard to suppress something as revolutionary & crazy as Crypto currencies.
Assume the same John Doe :), ordinary internet user, Never heard of XYZ Open source bank, No idea how it works, 0 control over it. Will John trust his life savings with this Open Source bank?
I'm not trying to de-motivate you or diss you, infact I encourage everything Open Source! It's just that I'm attempting to outline that something out there already exists which solves may be atleast some of the problems you want to solve. Good luck!