What, movies made in a country with about 1% of the population being white people star mostly Japanese people? I'm shocked.
You cite Idris Elba possibly playing James Bond. Why not? The man is actually English, unlike most of the actors that have played him. There's no reason not to. Generally I support colorblind casting, especially when it's for a work that there's really no reason for a specific ethnicity to be considered.
Ghost in the Shell is not one of those cases. It is a Japanese story with Japanese cultural sensibilities. Just about everything about Ghost in the Shell is based on Japanese culture. Casting Scarlet Johanssen as a character named Makoto Kusanagi in a story that takes place in Japan and is based on Japanese culture is blatant whitewashing.
Of course, whitewashing is the least of what I expect from Hollywood with Ghost in the Shell. I fully expect all of the complex themes of transhumanism and feminism as well as the exploration of society and technology to be cut out in favor of making just another action movie. And on top of everything, this is another unnecessary remake because Ghost in the Shell is already a film.