We begin the "supporting" version of this recasting project in the same year we did for lead. Mery's actual filmography began with 1977's Julia. But we know that she was doing auditions for films earlier, as she's famously described her encounter with Dino De Laurentis in her audition for King Kong. That film was ultimately cast with Jessica Lange, and was released in December 1976. So it's not too wild to assume she may have not only been interested in doing a film a year earlier than she did, but was getting auditions with established directors.
One of those directors could've been John Schlesinger, whose Midnight Cowboy had scored him an Academy Award for Best Director earlier that decade. Schlesinger reteamed with Dustin Hoffman in 1976 for an adaptation of William Goldman's thriller novel, Marathon Man. I'll say right off the bat that it's a bit of a stretch to think that Streep would've been considered for the role of a French and German-speaking Swiss woman for her first film. That being said, it's known that Schlesinger originally envisioned Julie Christie (a Brit) in the part of Elsa Opel that eventually went to Swiss actress, Marthe Keller. It's not a huge role and Keller was by no means a big star in Hollywood, but had up to that point appeared in several French and German films. Had Meryl secured an audition, we can expect that she would've been able to adeptly display her knack for accents and language, perhaps even to the point of being considered. We'll continue under that premise.
The film basically follows Babe Levy (Hoffman), a Ph.D. student and jogger, whose brother, Doc (Roy Scheider) is a secret agent for the U.S. government, acting as a diamond courier for a Nazi war criminal, Dr. Szell (played by Laurence Oliver), in exchange for information in tracking down other Nazis. The plot is pretty convoluted, so suffice it to say that Elsa is also an agent secretly working for Dr. Szell. She becomes Babe's girlfriend to get info on Doc, whom Szell is now trying to kill because he feels he can no longer trust anyone with the diamonds.