Drew Pearce (Iron Man 3) kicked off the script, before handing over to McQuarrie when the latter came aboard to direct. Given his long-standing, close working relationship with star and producer Tom Cruise, McQuarrie has naturally given the screenplay his own particular polish even as he figures out the logistics of the new movie.
Bringing Staples in would appear to be a way for the script to evolve while taking some of the workload off McQuarrie's shoulders. Staples has several screenplays in development, including a project for Ben Affleck (Runner Runner) to direct, the Jeremy Renner (The Bourne Legacy) vehicle King Of Heists and a movie based on King Tut. Until now, he's been better known as a video game writer, having worked on Need For Speed: Rivals and Call Of Duty: Modern Warfare 3.
Paramount is naturally hoping for another success from the Mission franchise, following on the heels of Mission: Impossible - Ghost Protocol (2011). The fifth film is set to hit cinemas on 25 December 2015 in the US and 26 December over here.