The highest priority feature that GIMP developers are still after is support for layer masks on layer groups, which dates back to a bug from 2001.
Some of the other high priority features the open-source graphics program developers are after that are currently a work-in-progress include cleaning up the GIMP library (libgimp), porting from GTK2 to GTK3 (there's still GIMP's gtk3-port branch), and high bit-depths.
Additional priority features are support for filter layers, script recording and playback, smart objects, layer effects, a unified transform tool, and better support for image metadata.
The features being hoped for in GIMP 2.10 -- the next major release -- include OpenCL support in GEGL, high bit-depths, seamless cloning, the unified transform tool, unified configuration path across platforms, auto-anchoring of floating selection, and the improved image meta-data support. GIMP 3.0 will come when the GTK3+ port is complete.
For those wanting to look at the feature road-map for GIMP either out of interest for this popular, multi-platform, open-source graphics program or are wanting to possible contribute to the project, check out the GIMP.org Wiki. Other ways to help out GIMP are also mentioned on the project web-site.