A proposal to add a currency token to an anonymous email system.  Each phone acts as its own email server running peer-to-peer between two phones.  A central registry maintains identity and IP addresses so phones can communicate.

So.  The biggest problem is that email is not a guaranteed delivery protocol.  In 2014, this system was losing about 2% of its emails.  No sane bank will sign up to lose 2% of its money.  After a few days, I defined the overall design as a guaranteed delivery system between mobile devices and started researching.

A second problem is scalability.  The client wanted to scale to 100 million devices.   So guaranteed delivery and high scalability.   Eventually, I looked at IoT solutions...

Other constraints - how well does the IoT toolkit match the existing environment?   Cost, effort and applicability are concerns.   I recommended the RTI toolkit as the best option with an annual licensing cost of $7500 per developer.  At that point, this project died.   There's initial sequence diagrams and even coding of the Bank Manager, mock tokens, unit tests.