This said, if anyone here would like to give it a shot... our hardware is open source so relatively easy to add both plugs, firmware should just work, and I'm happy to support with manufacturing. I would highly recommend to test the market first (landing page, or even crowdfunding campaign).
Or just go 'full compat' and do NFC/USB-A/C. I could see something like that being handy for IT folks.
* The A variant yubikey is crush resistant and mechanically simple for a longer life.
* The adapter acts as a form of connector saver for my yubikey, since I usually keep it physically attached to the yubikey itself. With this approach both wear-prone sides (C male and A female) are on the adapter, which costs as little as 1/50th the price of the yubikey.
You might know what you are doing, but are you sure that's true for everybody who might find and use it in the future?
Please don't buy/produce these things.