[crediblewitness]

Sam Altman has written about a US Digital Currency (http://blog.samaltman.com/us-digital-currency), and considers the idea that it's issued as a "second" legal tender. This IMF article also considers the idea of a "second" currency, but for other practical reasons (negative rates). I think it's actually somewhat inevitable that this happens. Whether or not it would have any cryptographic properties is up for debate, but there is certainly exists enough existing experimentation in this area for the Fed to study such an approach.