The Gunny was the Drill Sargent's Drill Sargent. There are roles that actors take which end up defining who they are in filmography. The Gunny did that for R. Lee Ermey — Stunning, just stunning.
As for mimicking the maverick quality of Shelby a Jonny Depp like personality could do some considerable damage
. The tough part is finding or integrating all the other extraordinary qualities the man had into a single actor.
Whoever they pick, I still want to see the movie.
Ed