I've been asked about the feasibility of an accessibility keyboard replacement, a system wide virtual keyboard. iOS seems to have such support via App Extensions | Custom Keyboard. Unfortuantely I can't seem to find something similar for macOS.
Catalina's System Extensions seems like something that might offer a glimmer of hope. However we are not dealing with a hardware device that will be "driving" the extension. "Activity" would be generated by a macOS app providing a virtual keyboard to the user. Hopefully a system extension could be used to pass these app events to the system so they appear to other apps as keyboard events.
Not only does the macOS app have to somehow enter keyboard events into the system but some other app has to retain input focus to receive them.
Such macOS apps existed in the past but I expected they have problems with the current security model.
In summary, there are at least two problem. (1) Letting other apps retain input focus. (2) Generating system keyboard events.