This is exactly my dream too, especially after seeing the Apple Watch prototype dumbphone cases they used to conceal them in public[0]. It would be a glorious re-purposing of the Apple Watch to serve the most diehard fans of the killed iPhone Mini.
Sadly the Apple Watch doesn't do proper external text input. You can connect a bluetooth keyboard, but it works by sending all input via the VoiceOver accessibility feature, which is slow and fidgety.
