New Yorkers don't buy newspapers like they used to, or cigarettes, probably because people are increasingly reading the news on their phone while honking on their Juul. That's an issue for newsstand retailers inside the subway, who often hawk candy bars and print news media—and that's partially why there's currently a lot of empty real estate within MTA stations, 40% of 326 total properties currently going unused.
Starting in the summer, the MTA will begin to roll out a wave of alternative options for vacant retail spaces, including pop-up shops and vending machines. In a statement, MTA Chief Development Officer Janno Lieber also cited "larger scale retail concepts" like the subway shopping options at Columbus Circle as part of the MTA's vision for train retailing, meaning that more sizable subway mezzanines could very well become underground mini-malls.
The MTA has explored pop-ups in retail spaces for several years now (Their pilot pop-up in 2013, ironically enough, was an "indy-oriented media seller" called The Newsstand, and was installed at the Lorimer stop.) MTA spokesman Shams Tarek notes that ahead of the new rollout, the MTA did outreach to figures in the private sector industry, including retail marketers, architects, and landlords, and solicited ideas about what might play well in these empty spaces.

A view of what the vending machines might look like, starting this summer (Courtesy of the MTA).
Tarek tells Gothamist that the vending machine offerings will be "driven by the market." They'll likely include goods such as tiny toiletries and snacks people can eat while wondering where the hell the C train is.
What subway stations will be the first to receive these newfangled vending machines is to be determined, and Tarek says vendor negotiations are under way. The MTA's proceeds from the vendors will "go towards running the system."
Tarek also notes that the vending machines will go in unleased and unused spaces. Existing retailers, including newsstand operators, won't be driven out by the near future of automated snack machines.