As much as people like to joke about his acting, Reeves is a good actor in certain roles. Considering Klaatu is an emotionless alien, I think he would excel in the role.

I disagree that plot is the most important part of a work, though it is important. Many movies have similar plots and many more borrow plots from other movies or works (think pretty much all Disney animated movies) but make them their own with characters and effects. Think of the numerous book to film adaptations and know that plot is only one element of a successful movie.