In Star Wars: Rogue One, they had a completely CGI'd version of Peter Cushing. Because the movie was set before the first Star Wars movie, Cushing's character needed to be in it, and Cushing has been dead for many years. So they CGI'd him.
They also de-aged many of the actors who flew against the death star as X-Wing and Y-Wing pilots in the original Star Wars movie. The Battle at Scarif was a really cool scene.
I doubt filmmakers could use an existing actor’s likeness without his approval. But they could do one better: invent their own. No more casting ; they would just design the characters the same way they design sets and costumes. They could even synthesize the voices.
At that point, “actor” becomes “waiter.”
They did not need to do that. They could have met him at the voting booth and signed him up for the gig right there!