For inspiration, check out the Vocal Eyes Becker Communication System: https://jasonbecker.com/archive/eye_communication.html
A system invented for ALS patient Jason Becker by his dad: https://www.youtube.com/watch?v=wGFDWTC8B8g
Also already mentioned in here, EyeWriter ( https://en.wikipedia.org/wiki/EyeWriter ) and Dasher ( https://en.wikipedia.org/wiki/Dasher_(software) ) are two interesting projects to look into.