EasyKey is a one handed keyboard with an older app version available on Google Play, based on US Patent 5,006,001. There have been many improvements since then.
Presently I am looking for a simple modification on the output of a number pad to make the cursor movement more useful (to slightly change the arrangement and make use of unused keys) and not be stuck with the limitations of the archaic layout in common use. Successful development could lead to further enhancements contracted out.