According to Dan Nykolayko, a member of the METALLICA staff, The follow-up to Death Magnetic is currently in its mixing phase.

Lars Ulrich recently explained in an interview with Metal Forces "We're now coming towards the end of the musical creative process and we're starting to look ahead and the process of how we're gonna share this record with the universe. The month of June is basically when we're gonna sit down and figure what we're gonna do with it all; what we're gonna call it and what's gonna be on it."
Lars also explained that the new album may not be released in 2016: "If the record doesn't come out this year, then it won't be because it's not done. It will be because there's some sort of cosmic reason that it would be smarter to hold onto it until next year. But the record will be done this summer."
Metallica Live the night before