There isn't yet any planned release date for the next version, GIMP 2.10, but features to be expected include finish porting the core to GEGL, support for high bit depths, support for layer masks on layer groups, cleaning up the GIMP library (libgimp), merging of GSoC 2011 projects, and work on a unified transform tool. That's what is officially hoped for at least with GIMP 2.10.
New activity continues to hit the GIMP Git master repository with the most recent work being just hours ago. With the 2.10 development, they are trying to do more work though in feature branches until the work is actually ready and then merging the feature branches to Git master, which will ideally leave the master code-base in a more sane and stable state and hopefully leading not to a long and drawn out release process if features linger around in an unstable state.
Some work to have recently been committed to GIMP master includes updated translations for various languages, bug-fixes, and massive work on the unified transform tool.
The goal for GIMP 3.0 at some point in the future is to port the program from using the GTK2 tool-kit to using GTK3. Like the Xfce developers, GIMP developers haven't been quick to port their massive code-base over to the major GTK3 release. (There does exist a GIMP GTK3 branch, but not that's been merged to mainline.)
Other features hoped for in future GIMP releases include improvements in text handling, automatic layer boundary management, filter layers, non-destructive editing, auto-anchoring of floating selection, script recording and playback, and smart objects support. This future work is talked about on their road-map Wiki page.