This link has some speculation. Abedin is the best guess for the travel companion; she's not in her early 30s today, but she was when Clinton was Secretary of State, which is the time frame of the emails. Mary Steenburgen is a candidate for the TV/movie star--Jack Wheeler
alleged a Secret Service eyewitness account of her and Hillary during the early Clinton Presidency--but I wouldn't call her "popular", so I'm wondering if it might be someone like Ellen DeGeneres, whom Hillary
did email and who
attended the Clinton fundraiser organized by George and Amal Clooney, along with other names who might be candidates. Have some theories on other possibilities but I'll save them until I have more information.