The story comes from the guys at Schmoes Know, who are wildly incorrect as often as they hit on something true. According to them, Chastain has been offered a starring role as Cruise's "right hand woman", which could be anything from a love interest, fellow agent, or both.
With a Christmas 2015 date already penciled in, Christopher McQuarrie is going to need to begin production by the end of this year. That means we should begin hearing some firm casting announcements soon. There's also the potential problem of Chastain's lead role in the Marilyn Monroe biopic, Blonde, which may shoot this summer.