Say what you will about the Nolan Ryan or Randy Johnson fastballs, but Rivera's cutter is the pitch that changed the game. From about 1998-2008, pretty much every pitching coach from high school to Triple-A ball tried to find the next kid who could master it, but no one will ever quite have the success that Mariano did with it. Huston Street practically won a CWS for us with a similar pitch, and his version of it is pittance compared to Rivera's.
I don't think you'll see any more dominant "one-pitch" guys throughout the upcoming years in baseball, unless someone else can master Rivera's pitch.