A fingerprint sensor isn't possible, appreciated, but what about a U2F activation button? It could even just be a wireless pass-through for USB to allow your yubikey to work for example (I'm certain this can be done on Linux but unsure about other oses)
https://github.com/zimengxiong/tinytouch spoofs as a card reader but won't work with Apple Pay and the like.
> It could even just be a wireless pass-through for USB to allow your yubikey to work for example
Everyone would be better served with a generic USB hub in the keyboard, not shipping a security key with the keyboard.
My favorite keyboard for years was the Dell SK-8135. It had media buttons without messing with your function keys, a volume knob, and two USB ports!
For a long time I would argue (as an IT support person who would have to support your wireless keyboard problems) that having a wireless keyboard was wasteful and unnecessary if you never used it more than 3 feet from your computer anyway.