Identity-politics purists insist that it’s offensive for actors to play characters of a different race, gender or sexual orientation. So here’s a poser: Whom do you cast to play a character who self-identifies as a different species and becomes disabled in the process of her transition?

That’s not exactly the debate surrounding Disney’s forthcoming live-action adaptation of “The Little Mermaid,” but it could be. Halle Bailey—a black actress and R&B singer—was cast as Ariel, though Disney has historically depicted the Little...