The novelization had his last line be in Rey’s head after he dies: “Noones ever really gone.” Not only is it a nice callback to Luke, but it also explains why Rey suddenly looks happy as she’s flying away in her X-wing.
Too bad he never said that anywhere.
I like this idea. Ben’s last words could be something similar to Obi Wan’s - ‘Remember, the force will be with you. Always.’
It’d be a nice call back not only to his name sake but to the movie it all started with.