The unidentified role was reportedly offered to Chastain some time ago, but scheduling conflicts led her to decline it. Marvel had not found anyone to replace her, so when her schedule cleared up, Chastain apparently returned to the top of the list.
Early speculation seems to center principally around the character of Maya Chastain, a sexy scientist who was featured in the Warren Ellis-written "Extremis" story upon which the movie is at least partially based. If so, you can expect to see more of her: in the comics, the character eventually joined S.H.I.E.L.D. which, if you remember, plays a substantial role in the Marvel movies.
In order to accommodate the fact that she's still the female lead, it's being reported that Gwyneth Paltrow's role will be "beefed up." One has to wonder if she might don the Rescue armor...!