Had he not been cancelled - a Star Trek return?
Maybe in ST Picard s3 somehow? connecting V'ger (and Will Decker/ilia) into the borg for added fan service (couldn't very well do a callback to V'ger after Collins scandal bc it would call into question 'wheres Decker?')
Or Discovery (set in 32nd century maybe s5)
Or even the movie Star Trek 3 Beyond (as Commodore Decker , the father of the character he played in ST:TMP.).
Or as Decker snr for the Star Trek SNW series
it was feasible bc he was still a working actor in his late 60s appearing in TV and movies and looked fit/in shape (his Ted 2 scenes were cut after his scandal). and he was obviously on JJ Abrams radar as he was in one of his produced series 'Revolution'