GIMP's Progress In 2016, What's Ahead For 2017
Some of the highlights shared by Alexandre Prokoudine for GIMP in 2016 included better handling of layers / channels / masks / paths, better remembering of defaults, improved color management, tooling improvements, a plug-in to load darktable RAW files, WebP file format support, new themes for GIMP, greater use of GEGL and babl, and much more.
Among the work still being done before GIMP 2.10 is released includes cleaning up libgimp, changing linear/gamma-corrected workflows, and 16/32-bit per color channel support, a new color management implementation, and more. GIMP 2.10 will hopefully ship later in 2017.
But not making it for GIMP 2.10 is the port to the GTK3 tool-kit, which will then usher in GIMP 3.0 when completed.
Those interested in the progress made to GIMP and some of what's coming up can read the post in full via GIMP.org. Other feature work can be found documented via the roadmap page on their Wiki.