It'll probably focus on the aftermath of G'14, with the world coming to terms with what happened during the events of the film and the fact that giant monsters exist. I can see them bridging the gaps between G'14 and KOTM, giving more details about what happened in between those films, with Monarch tracking Godzilla, discovering the locations of other Titans, and possibly dealing with smaller scale incidents involving other monsters. I could even see them including the events of Godzilla: Aftershock, showing it from a different perspective (which would help to include larger scale monster fights, without having to retcon what was established in KOTM).
"Neither beast nor man. Something monstrous."