If Annapurna Pictures (the production company that owns the rights to “Her”) made a sequel where the voice AI is played by someone else than Johansson but sounded the same and was marketed as a direct continuation, I think there would be a lawsuit.
She didn’t write the script or develop the character, but I think there’s enough creative authorship in her voice portrayal that it would be risky for the production company.
Reading all these musings here about a possible "there is no bad publicity" approach, I'm starting to wonder if the plan for if Johansson signed up was achieving court drama publicity by getting sued by Annapurna Pictures. "Can a three-letter tweet be the base of a copyright case?"
In iron man 1, Rhodey is played by terrance howard. For iron man 2, he wanted too much money in the contract, so they replaced with with don cheadle.
Wouldn't it be a dumb world to live in if a single actor in the cast can halt the production of a new work via lawsuit because they own the character?