In testing now is an entirely new dynamical core for the US Global Forecast System, developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and known as the Finite Volume Cubed-Sphere Dynamical Core, or FV3 for short. See https://www.gfdl.noaa.gov/fv3/ for gory details.
In addition to a new dynamical core, the FV3GFS is also using the GFDL cloud microphysics parameterization to simulate cloud process. This should be an upgrade compared to the old GFS cloud microphysics scheme, but I'm not sure if it has been tested much in winter storms or complex terrain.
From a research standpoint, I'm not that interested in model performance during the warm season (i.e., right now and during the summer). I am, however, hoping to get my paws on some of the runs from this past cool season so we can see how it does for winter storms over the west.
Whether or not this proves to be the game changer that NCEP hopes it will be remains to be seen. I suspect we will see the FV3GFS become the operational GFS at some point later this calendar year or in early 2019.