How can one create a modern adaptation of Shakespeare's Macbeth?
Because Shakespeare's plays are so powerfully written, and the stories so enduring, many theatrical productions have sought to decontextualize them for different historical eras. This makes sense because many of the problems and situations are timeless, and portraying the plays with period dress would tend to fossilize them in the 16th century when most of them were written. For example, I have seen a production of Much Ado About Nothing set during World War II, and some of Kenneth Branagh's films of various Shakespeare plays have successfully used modern settings and costumes.
Macbeth is at its heart a play about a troubled, corrupt monarch who allows superstition to make him paranoid, and lets his wife manipulate him into committing murder. These situations can all be applied to a modern or contemporary context. The witches, for example, could be portrayed as either supernatural or imaginary beings, or as fortune tellers, or as young Puritan servants (a reference to The Crucible, itself a play using the Salem witch trials as an historical metaphor for a contemporary situation: the McCarthy hearings), or as new age shamanic healers, depending upon the historical context of the production.
Updating Macbeth's status as a monarch is a simple change: he can be a high-powered politician or famous celebrity. His vulnerability to belief in the supernatural or his wife's manipulation can be explained by any number of modern-day psychological disorders, or simple job-related anxiety.