I was able to build my latest semi-practical gizmo, a wireless keyboard that never needs to be plugged in. It's not Bluetooth - I wouldn't be able to get the power consumption low enough (I tried). By combining the ultra-low power abilities of a lower-level 2.4ghz wireless protocol and photovoltaic energy harvesting capabilities, this particular keyboard shouldn't ever need to be plugged in. Even in a cave the 500mAh battery should last 10-12 months – SouthPawEngineer.
It does require a USB receiver but it is QMK programmable.
The board features hot-swap Kailh sockets with Kailh Choc Brown switches and MBK keycaps. The 5 way switch in the center can be used with your thumb and it allows you to press it up, down, left, right, or in - and you can assign different functions to each direction as you normally would within QMK.