Economists at the Richmond Fed (Romero, Wang & Wong) have been thinking about them (as have many others):

Physical currency in circulation is a liability of the central bank. So most simply, central bank digital currency is central bank liability issued in electronic form. In a sense, the Federal Reserve already offers a digital currency in the form of electronic central bank deposits, also known as reserves. One might think of these reserves as wholesale CBDC since access to them is limited to qualifying financial intuitions. For retail payments, central banks have avoided dealing directly with the general public, relying instead on tiered arrangements in which commercial banks provide direct retail payment activities and services. At present, the only direct connection between the public and the central bank are the Federal Reserve Notes (paper money) people carry in their wallets. However, new technological advances, such as distributed ledger technology and mobile computing, have made it possible, on the one hand, for private parties to develop payments systems that bypass central banks and, on the other hand, for central banks to provide new forms of retail payments that bypass intermediaries.

This article was an excellent way to quickly catch up on the issues I quite frankly haven’t kept up with. Other articles of interest:

Joint report by The Bank of Canada, European Central Bank, Bank of Japan, Sveriges Riksbank, Swiss National Bank, Bank of England, Board of Governors of the Federal Reserve and Bank for International Settlements.: https://www.bis.org/publ/othp33.htm

A survey by Auer, Cornelli & Frost: https://www.econstor.eu/handle/10419/229473

And a recent guest post on Econbrowser.