Among the interesting development efforts going on for this open-source alternative to Adobe Photoshop is the development of a new project file format. GIMP has used its XCF as the primary project format since 1997 and now nearly 30 years later a new format is on the horizon.
GIMP's new project file format will rely on a zipped XML structure and will allow for more efficiently handling of file operations. Plus it will allow auto-saving and other improvements moving forward along with better handling of large and multi-page projects. Animation features planned for GIMP 3.6 will also work out better with the new file format.
This new file format remains under design and being implemented, but GIMP developers do intend to retain backwards compatibility with XCF moving forward.
Aside from the new file format, GIMP has been seeing new work on brushes, the non-destructive filter code has been seeing more enhancements, Adobe PSD support enhancements, and native file chooser dialog integration.
More details on this GIMP feature work via GIMP.org .