One could theorize that the first spot meant for Zhen Lei’s expertise in the Mission: Impossible series went to Jane Carter (Paula Patton) in Ghost Protocol.
However, we absolutely know that Ilsa Faust (Rebecca Ferguson) picked up that gig in Rogue Nation, as it was revealed that both Patton and Maggie Q were unavailable for the shooting of that film.
The latter case is particularly interesting, as Ilsa has become another fan favorite, with her place in the IMF team secured in every sequel starting with Rogue Nation, and continuing through the current two-sequel shooting plan.